yz400e wrote: a long bed would be on a frame with the longer wheelbase and hence a longer frame.
This is correct. The long bed adds about a foot of length in the bed in front of the wheelhouse. 7' bed and 8' bed. They are the same frame rails I recall; to make a SWB truck the additional foot is lopped off the rear of the rails and the axle mounts moved forward. The bed is shortened the same amount between the cab and the wheelhouses.
Based on what was said, I will stay away from the older frame and the 84. 78 - 79 seems like an ideal year as it would also get me drivers side drop..which may give me more options if I ever figure out what I want to do.
Somewhere in the late '60s the wheelbases (and resulting bed lengths) changed. Not sure about a '69, but it would be Buick powered (or AMC 232) and closed-knuckle front end, and I believe the front axle is also spring-over. AMC made steady changes in these vehicles between 1970 and 1974, so the list would be pretty long if you had to list every little thing that changes year to year.
These trucks switched to driver's drop in 1980. A '76-79 will be the same passenger drop as your '76 and the same type of spring mounting.
Regarding the '84, if you want to switch to driver drop, you could maybe switch the cab and bed from your '76. The floors change in 1980 to fit the changed drivetrain, so you'd likely have to modify them some. The '84 is pretty rusty, which should bring the price down a lot. With a bad engine and rust, I would expect to pay very little, like scrap prices plus? $2000 is fantasy. Both are missing a tailgate, which can be hard to source.