The GMC and Chev use different linkage. I would isolate the problem by pulling linkage apart piece at a time. Unhook it at the shift lever at the transmission and make sure that the trans shift lever moves into all positions. Bolted to the steering column under the hood is the retainer that the neutral safety switch bolts to. It also has detents for each gear position. The shift handle inside the cab has the standard action where the lever pulls towards you as you shift gears. That is what releases the stop lever from the detent positions on the bracket under the hood. My guess is that is where your bind will be in that it is not fully clearing the detent positions.

If you follow the links that Tim posted, you will find 7 booklets that deal with the Chevy truck hydra-matic transmission and linkage for 54. Keith Hardy has done a great service in scanning and posting a wealth of information available to all.