unless the headders are off a 351m or a 400, then they won't fit your 302 or 351W.What size motor in the '90 and the '81?
302 to 302 header swap will work no problems
302 to 351 will not work (302 header is shorter)
351 to 302 will work but y-pipe will hang low
351 to 351 will work with no problem
This all depends on what type or headers (long tube or shorties) you are running and also what kind of exhaust you are planing for.