GMC offered an automatic in '53, Chevy did in '54. If you go year by year there are likely a number of things like that to consider.
GMC offered trucks larger than the 2-ton early in the 50's while Chevy did not until near the end of the decade. Those larger trucks will show a number of differences from the Chevrolet trucks including some huge six cylinder motors and similar looking but different body panels.
When you get to the Task Force/Blue Chip trucks the range of engine differences become more extreme. While the standard motors are the Chevy 235 and GMC 270 the V-8s are the Chevy 265/283 while the GMC trucks had the much larger Pontiac and Olds motors.
I've seen GMC trucks that appeared to have original Dana axles just as early as open drivelines appeared and those axles had some ratio options that Chevy did not offer.
As Ken said a lot things will interchange, but if you are trying to build an accurate restoration some things are very different.
