As mentioned above: Before attempting to install the muffler pipe to the manifold it is a good idea to loosen the clamp at the rear of the pipe.
Also Service Bulletin page 327 covers a new style clamp and a new hole that is drilled in the frame to correctly install the new clamp. There are two diagrams showing the new clamp and where to drill the new hole.
The larger vendors have a Repro of the new style clamp.
Many cars still have the old style clamp and the holes in the frame are wrong.
Also the front clamp is in two pieces.
1, It is possible to install the clamp upside down which will allow the muffler to fall off.
2. The clamp can also be installed with one half right side up and the other upside down. In this case it will hold better than step "1." above, but may fall off.
If you look at clamp with the bolts installed and not on the muffler or manifold the top of clamp is slightly larger inner diameter than the bottom.
Some of the cheap clone mufflers have a flange that is too small and the clamp can not hold the muffler correctly.
The Aries muffler is the best fitting Repro muffler.
http://www.amuffler.com/products.html