Ineresting...In August, I installed a single exhaust system purchased from NPD in a 67 coupe. The Y-pipe, intermediate pipe, and muffler fit perfectly. Used the original muffler brackets and everything aligned up properly.
Maybe this is one of those your mileage may vary things wit the parts vendors...
To be factory correct, your tail pipe will need to be welded to the muffler. The intermediate pipe is clamped at the Y-pipe and at the muffler inlet.
Regards,
Pete Morgan