There was a thread I was reading about interchangeability between the doors. Our square trucks really divided into two body styles. Someone was asking if all the doors interchange and they do, but after they measured the length of the door everyone was saying the new style doors are barely one half in longer.
Put some of this in your pipe and smoke it !!!
73-76 ALL PICK UPS& SUBURBAN-SAME PART#
76 BLAZER & JIMMY HAVE THE SAME #
77-87 PICK-UPS &77-90 BLAZER & SUBURBAN-SAME#
And I wouldn't be suprised if the differance in the 76 to 77 doors, might be some added holes in the assembley of the core of the door. Now I am not going to swear to the that. But I also beleive 77 was first year power windows were optional, so that is likley why the differant part #. I know I'm close if on target. Retro can verify exact year model I'm sure.