Is a GMC truck just a dressed up Chevy truck?

The early hemis were in dodges, desotos and chryslers. They were totally different motors.
Ford and mercury had similar flathead motors. Mercuries had a few more cubes and horsepower.
Some early GMC trucks had pontiac motors in them [straight 6 cyl. flathead with 2 cyl. heads.]
Up to the mid 1970's all gm products had different V-8 motors. Buick, olds., pontiac, chevy, and cadillac were all different. Mercury had a 410 which was an overgrown 390 ford. Lincoln had its own flathead and its own 462 V-8. There was a lot of stuff happening back then.