The decision to essentially merge GMC and Chevrolet trucks in 1968 had several repercussions. Chevrolet would have access to GMC’s HD truck line (and engines), rebadged as Chevys. And reciprocally, GMC would start selling light and medium duty Chevys as GMCs. This was the beginning of the end of what had been two truck makers that mostly had shared some cabs, as well as pickups and light vans, but with mostly unique engines.
This GMC ME-6500, technically a medium-duty truck, is hauling a good sized load and is all-Chevy, except for the badges. And the badges on its sides proclaim that it’s got a Chevy 427 V8 instead of one of GMC’s big V6s.
As a frame of reference, GMC’s “own” conventional medium and HD had this cab, which shared nothing with the Chevy.
It came in short nose and long nose versions. We’ll do a more detailed post on them one of these days.
Back to the Chevy-
GMC: I should point out the 366 and 427 Chevy Mark IV truck V8 engines were not exactly the same as those used in Chevy cars and light trucks. The biggest difference was that these were tall deck engines, meaning the block deck height (top of block) was higher, so that longer pistons with four piston rings could be used. That improved durability at the constant high loads these engines were subjected to.
These are referred to as “tall deck” 366s/427s. And there’s other differences of course too, from sodium filled exhaust valves to heavy duty components all-round, and of course the accessory drives are set up for truck use, for air compressor and such. The Holley four barrel has a governor, which works off the mechanical advance distributor, limiting max. revs to 4000. These were of course tuned for maximum torque and continuous power output, and at full chat, exhaling through two big short pipes, they were somewhat famous for their vocalization. And their prodigious thirst. The 427 was rated at 260 gross/ 230 net hp.