Yes, the bumpers will interchange from 1975 through 1979 and are the same for all body styles. Although there may be some trim differences (some have rubber impact strips and/or rubber bumper guards).
The seats will also interchange from 1975 (and earlier) up through 1979 but they are more body style specific. For example, front bench seats for 2-door & hatchback models have split backs that fold forward for access to the rear seat while 4-door front bench seats have a fixed (non folding) solid back. And the rear seats are also specific to each body style.
About the only thing that is considerably different between 75 and 77 is the dash, instrument panel, and associated wiring.