GRANDE MAX

Stan and Lee Ann Pitman’s Duramax Powered 1970 GMC 2500 Sierra Grande

Stan and Lee Ann Pitman are a lot like your typical couple but with a few twists. They own S.D.P. Manufacturing in Dunkirk, Indiana a manufacturer that builds equipment used to install utility poles. They specialize in machines that are easy to transport and will fit through small openings for residential yards. The twist comes in Lee Ann’s choice of transportation. Like many suburban women across the country, she chooses to drive an SUV, but her SUV of choice is this restored ‘70 GMC Sierra Grande 2500 with a modern chassis and Duramax/Allison drivetrain below the classic three-door. What’s more, she searched for and eventually found the restored SUV online, flew down to Orlando, Florida and drove it back home to Indiana (with a failed starter replacement along the way) then commissioned the conversion of the GMC. Her new ride went from a lifted monster with a weak 350ci gasser under the hood to a more useful height with a durable modern chassis and powerful diesel drivetrain.

The previous owner restored the Sierra Grande, giving it a fresh two-tone paint job with Baby Bird Blue accented by Bright White on the roof and rocker panels. So when Lee Ann handed the SUV over to 20-year-old Mason Wright at Wright Way Collision Repair in Albany, Indiana he didn’t have to mess with the paint and body other than giving it a final detail with a clay bar and polishing the finish. That was just the beginning though, as there was still a lot of work to be done to complete her vision for the GMC. To get things started Wright removed the body from the chassis and then went to work adapting an ‘05 Chevrolet Suburban 2500 chassis to fit and work with the classic body.

After rolling the old chassis out from under the body Wright examined the mounts on both the body and the donor ‘05 Suburban 2500 frame to determine how the mounts could be used/reused to mate the two together. He found that all twelve of the mounts on the frame needed to be removed, modified, and relocated to span the 35-year gap between the 1970 body and the 2005 frame. Additionally, he had to move the rear axle forward 3.5 inches to align the wheel wells with the chassis when the body was installed. Relocating the rear axle also necessitated moving the Suburban’s fuel tank forward 3.5 inches. After test fitting and verifying the proper alignment and clearances for the body and chassis he also had to make sure the Duramax engine and Allison transmission would properly fit in the classic sheet metal.

Once everything was welded and fabricated to make joining the chassis to the body possible Wright rebuilt the suspension to make everything like new for Lee Ann. In the rear this included installing a leaf spring pack from an ‘06 Silverado 2500 HD with Bilstein 5100-series shocks to tame the ride. The freshly painted rear axle assembly sports the stock G80 posi-traction differential and gearing but is capped with a cast aluminum PPE cover. Up front he installed new Moog control arms along with new CV axle assemblies and another set of Bilstein 5100-series shocks. All new hardware was used to bolt everything together rather than trusting anything to chance for the build. To make sure the Sierra Grande would stop like a modern truck Wright installed all new brake lines as well as new rotors, pads, and calipers giving the 50-year-old rig modern stopping power. Then to wrap everything up he slathered the chassis and components in gloss black paint for a finished chassis that looks better than new.

The next step in the build for Wright was addressing Lee Ann’s desire to have a Duramax engine and Allison transmission for her GMC. He purchased a wrecked ‘06 Chevy 2500 HD farm truck to use as the donor that would supply the engine, transmission, transfer case, electrical harnesses, and control modules needed to make everything work together in the GMC. While the LBZ Duramax engine is stock, he did dress it up and tune it for improved performance and better fuel mileage. To dress up the engine he removed the accessory drive brackets, valve covers, front cover, and turbo compressor housing then thoroughly cleaned and degreased them before painting them bright blue to give the diesel engine a pop of color. He also replaced the factory intake and boost tubes with higher flowing and better-looking units from Wehrli Custom Fab that were powder coated in a rainbow flake dark gray color.

Several new parts like the alternator, AC compressor, CV axles, tie rods, brake calipers, and power steering lines that were replaced and installed during the project were treated to a clear coat paint finish to prevent rust and keep the parts looking like new for many years to come. For improved longevity for the engine itself Wright installed a set of ARP head studs to keep the heads sealed to the block for many miles to come.

An AirDog II-4G 165 fuel pump and filter system feeds plenty of #2 to the stock Duramax CP3 high pressure fuel pump from a Suburban gas tank that now holds diesel fuel. Wright improved the exhaust flow with a set of up-pipes and exhaust manifolds from PPE that feed into the VGT turbine of the stock LBZ Garrett turbo charger. From there spent gases are sent away from the engine through a 4-inch aluminized steel MBRP turbo-back exhaust system that exits in the stock location behind the passenger side rear tire. The subtle upgrades combined with EFILive tuning that Wright performed on the engine give Lee Ann an estimated 450 horsepower and 950 lbs-ft of torque to motivate the GMC Sierra Grande–much better than the old small-block Chevy gasser that was in it when she bought it. To keep the engine and transmission cool under operation Wright was able to integrate a factory core support complete with the radiator, intercooler, and transmission cooler directly into the 1970 GMC front end so everything operates like it would for any late model Duramax pickup.

The Duramax engine is backed by an Allison transmission and NP261XHD manual transfer case to send the power to the front and rear wheels when needed. With the relocated rear axle assembly in the chassis Wright had to cut and reweld the rear driveshaft shortening it by 3.5-inches to get everything to mate up and align properly. The Sierra Grande rolls on LT285/70R17 General Grabber A/Tx tires that are wrapped around American Racing 17×9-inch wheels on all four corners for good traction on any surface.

Before handing the Sierra Grande back to Lee Ann, Wright gutted the interior and applied Dynamat Xtreme sound damping material throughout the cabin to give the rig a quitter ride like modern SUVs. He then installed a new carpet kit from LMC Truck before reinstalling the restored seats and Kenwood stereo system. The donor gauge cluster is integrated into the factory dash panel and the donor steering column along with a new leather-wrapped steering wheel help Lee Ann comfortably point the rig in the desired direction. Additionally, he made sure that both the front and rear AC were fully functional to keep the large interior cabin comfortable on long road trips no matter what the weather is like outside.

“Lee Ann’s Sierra Grande 2500 looks great from any angle.”

GM is finally building modern full-size SUVs with an available 6-cylinder Duramax diesel engine, but Lee Ann wanted Duramax V8 power combined with a classic body style that stands out in the crowd of cookie-cutter SUVs. She found her dream truck then turned to Mason Wright to make the dream a reality by converting it to a modern chassis with Duramax power under the hood. She uses the Sierra Grande mostly for weekend getaway trips and things like that, but it is built to handle anything she would like to throw at it—making it the ultimate SUV!