[Brentwood Bob]

Pinchy the Great: Hope you got some help here. The angle of the inlet side of the muffler pipe into the muffler is important to get the muffler to hang correctly by the hanger on the frame above the rear axle. If you don't have a go-to muffler guy, I would suggest replacement. The Aries muffler is the best choice. It is available direct from Aries, or thru the model a parts suppliers. Built to original specs, and sounds like the original also.
I have had to take a ball pean hammer to the cheap reproductions from the swap meets in order to improve the fitup to the exhaust manifold.
Use a floor jack under the muffler assembly after hooking up the rear mount loosely to the frame as mentioned above.
The floor jack frees up your hands so you can do this job yourself and concentrate on the muffler clamp. I use a small ball peen to tap the clam-shell as I fit it up and alternately tighten the brass nutted clamp bolts snug.
Bob, AKA Pinchy fume.