Need some help... new TTI exhaust wont fit

Sounds like maybe the H pipe is upside down?

If the H pipe is hitting the floor, that would be my first guess. OP needs to flip it and try that way.

Also - agree with having to shorten the H pipe for 108 inch A bodies - i.e. 67-69 Barracudas, Dusters, Demons and Dart Sports and 67-76 Valiants except Dart-based Scamps. Darts had a 111 inch wheelbase (except for Duster-based Demons and Dart Sports - clear as mud, right?).

Also - absolutely agree with those who suggest that the seat belt bolt is not where you hook the muffler hanger. I put TTI step headers and a 2.5 inch X-pipe exhaust in my 67 Barracuda and once I shortened (cut) the X pipe at the muffler end it all fit perfectly. But I already had installed a reproduction of the factory style passenger side muffler hanger bracket (car has had dual exhaust since 1991). As I recall, even though this was originally a six cylinder car, there were indentations stamped into the passenger side floor for locating the bolts for the passenger side hanger bracket. Just drill the holes, bolt up the bracket and go.

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PS - I've put a new bumper, new shocks and larger rear tires on it since I took this picture.