The '75-'76 heat exchangers were different from the earlier years. The SS exhchangers are available for either 2.0 muffler or 1.7/1.8 muffler. The difference being the 2.0 pipes are straight at the end and the 1.7/1.8 have a bend up. Plus, the distance between mounting points, once installed, is different.
Look at your SS exchangers to see what kind they are - straight = 2.0 and bend = 1.7/1.8. You will then have to get the appropriate muffler to match.