A 69-72 body will not fit without some modifications, those are the first gen's. The 73-77 bodies will bolt right up to the '76 frame. When swapping second gen bodies the big year to remember is '78. '78 was the year the rear footwell was intro'd, along with the correspondingly different frame (in that area)
You can make the early second gen body fit on a later frame, you just need to create one set of body mounts in the footwel area. A post '78 body on an early frame is a ton harder though.
Rene