‘74 gremlin bumpers

If you are planning on retaining the 74-76 model bumpers themselves, tucking the front in is fairly straightforward, but does require some effort. The bumpers are on shock absorbers, which will need to be drained of fluid, compressed to the depth you want, and then spot welded into that position. you'll get 3/4" - 1" of tuck before the bumpers hit the fenders.

The rears are a bit more involved, but there also isn't much distance that you can tuck them, as you'll wind up with the bumper ends rubbing against the sheet metal at the wheel opening flare.  AMC dimpled the rear quarters where the bumper ends are for this very reason.  Also, the rear bumper struts are rigid, so you'll need to cut and weld them to a shorter length.  A good bit of work for maybe 1/4-1/2" of tuck - probably not worth it IMHO.