Building a ’55 Chevy with Duramax Power

We talk to a lot of people who want to put diesels in muscle cars, and we’ve even run across a bunch that have pulled it off. Recently, we got to talking to Harvey Grant, who is right in the middle of taking a classic ’55 Chevy and fitting it with Duramax power. While he initially was looking at going with an LS gas engine, Harvey got tired of seeing the same old cracker-box cars show after show, and he decided to put a Duramax in his ’55. Since he’s already built an ’87 Ramcharger with a 1,100-hp 12-valve Cummins, he decided to take the plunge into the electronic world with his ’55 Chevy.

A Great Starting Platform

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There are a lot of hurdles when it comes to squeezing a modern three-quarter to one-ton truck engine into a classic, mainly spatial. Fortunately for Harvey, the ’55 he started with has a very tall engine bay that’s helpful with hood clearance, a wide frame that can accommodate a diesel engine and transmission, and a vehicle that has plenty of aftermarket parts available in case he runs into trouble. While he’s far from finished with his classic Chevy project, he was nice enough to share with us his thoughts and ideas on the build so far.

A Better Chassis and Body Swap

After Harvey ran across a used and abused ’55 Chevy frame at a swap meet, he decided to buy the frame with the intention of building the stock chassis for the diesel drivetrain, and then laying the body on top of it. While this does have its advantages, Harvey has no delusions when it comes to placing the original body on the new frame.

“Doing the frame off means I’ll be able to get a running rolling chassis before the body is even on the car,” Harvey says. “Unfortunately, from there, I know I’ll have the body on and off a number of times before it’s complete.”

Perhaps one of the biggest decisions in building the Duramax ’55 involved using a Kimbridge Enterprises’ weld-in front clip, designed to convert the original suspension over to a GM A-body design. This would allow for the use of more modern suspension parts, as well as front steering.

Here, one can see the advantage a front-steer setup offers as far as clearance is concerned. Still, the engine had to be moved over one-and-a-half inches to the passenger’s side for clearance.

The front clip came with a tubular radiator core support, which was helpful in locating the engine. Harvey installed the ’04 Duramax as low and forward as possible, for hood and firewall clearance. “It should just barely fit,” Harvey notes.

“I started making my own engine mounts, but then I decided to just Google, ‘Duramax engine mounts,’ and these came up on EBay,” says Harvey. The engine mounts were then welded onto the Kimbridge crossmember to locate the engine.

Not only did moving the engine over give steering for clearance, it also allowed the large Allison five-speed transmission enough clearance to be installed—filter and all.

Although Harvey kept the factory frame, he did modify it. The rear of the frame has had the leaf springs moved in parallel with the frame for tire clearance, a rear crossmember added, and a complete GM G-body front clip from Kimbridge Enterprises installed. Harvey explained the decision to use the G-body front end by saying: “It’s a more modern design that you can get parts for, at any parts store. It’s also a front steer, which creates more clearance than the original firewall-box design. It’s stout enough to handle street driving, and strong enough to support the weight of a diesel, which was also important,” Harvey continues.

Taking a Risk on a Donor Truck

“It took some time to find a drivetrain,” Harvey mentioned. “While it would be nice to get a complete running engine and wiring harness on a pallet, not many people have the budget for that,” he says.

After a few months of searching, he ran into a well-used ’04 GM pickup that had been hit hard on the front end. The insurance company had offered the guy $1,500, so that’s what the guy had been asking. Since the damage was in the front end and extended into the engine, prospective buyers were shying away from the project.

“When I first got the truck, I didn’t even know what parts were still good on it—it was definitely a risk,” Harvey says. As it turned out, most of the engine was reusable, the block and cover weren’t cracked, and it made a great LB7 starting platform for the project.

The exhaust is pretty tight on the passenger’s side, but the downpipe looks like it should snake past the ’55’s stock frame. Also, notice how much ground clearance the engine has, even with the factory oil pan.

Harvey didn’t like the looks of the high-mount, factory accessory drive, so he moved the alternator down below the belt tensioner, and the A/C compressor off to the side. Not only will this result in a cleaner-looking engine bay, but in more hood clearance, as well.

One of the hardest parts of a swap like this is the wiring, so it was fortunate Harvey bought a complete donor truck. With the help of Rick Fletes, who has wired up a Duramax Chevelle, the factory wiring harness will be stripped, then programmed with a 450rwhp EFILive tune.

Harvey fabbed up his own ultra-thin transmission crossmember to make sure there was enough room for a four-inch exhaust. The crossmember also has internal bracing to deal with the weight of the Allison 1000.

The rear suspension was left mostly stock, except for a spring relocation, and a set of CalTracs traction bars.

Swapping in some Duramax Power

The ’55 has a wide chassis, which definitely works in favor of a diesel swap, but there was still quite a bit of trickery involved in getting everything to fit.

“The front-steer did solve a lot of clearance issues, but I had to move the engine about an inch and a half to the passenger’s side to clear,” Harvey says. “Using the mounts from the Kimbridge core support, I also mounted the engine as far forward as possible in order to give enough firewall clearance,” he continues. With the engine moved over, that meant that the transmission crossmember also had to be slightly offset, as will the rear axle.

“I ended up with a ton of money in the rear axle on my Ramcharger,” Harvey notes. “… so I wanted this one to be as budget as possible.” To that end, a Ford nine-inch rear end out of a truck was chosen, for its parts availability, 3.0 gear ratio and strength.

There’s no doubt that the ’55 will hit the track after it’s running, as evidenced by the large 325/50R15 BF Goodrich drag radials mounted out back. With a 28-inch tall tire, a 3.00 rear gear, and 0.71 Overdrive in the Allison, the Chevy’s theoretical top speed should be well into the 150+mph range.

Harvey’s no stranger to rear-axle breakage in his Ramcharger, so he’s already back-braced the junkyard nine-inch. The axle is to be narrowed, fitted with 3.00 gears, a locker, upgraded axles, and drum brakes from a Ford pickup to help hold the big diesel at the line.

Once the frame is complete and running, the body will be taken off of this existing ’55 Chevy and then mounted on the diesel-powered frame.

For those who are worried about cutting up a rare ’55-’57 Chevy for a transmission tunnel, not to worry, the factory floor pan isn’t in the best of shape. Harvey plans on leaving the body and interior all classic ’55.

If you look at the height of the factory small block in the cavernous engine bay, it’s easy to see why the ’55 makes for a good swap candidate. The battery will need to be relocated to the trunk however, to make room for the side-mounted A/C compressor.

With the goal of an 11-second, 30mpg classic, we’d say Harvey is well on his way to creating the diesel muscle car of his dreams. Check back with you next year, Harv?

The Road Ahead

“I have a lot of work left, that’s for sure,” Harvey says. “We’ll see if the body-off frame thing was a good idea when I start putting everything together. In the meantime, being able to look at a complete Duramax-powered rolling chassis will be great motivation for the future. I can already see a completed 11-second 30mpg muscle car in my head—now I just have to get there!” DW