Pamcopetes solution for slipping on aftermarket Mufflers.
Extract from a Pamcopete post;
"If your stock headers are double walled, the functional pipe is inside the outer pipe, so if you are going to use the stock headers with an after market muffler, you need an adapter that will connect the muffler to the smaller inner pipe. And, yes...the outer pipes do have drain holes in them, so if you just clamp the muffler to the outer pipe, the exhaust will leak out of the drain holes.
Here is a pic of the inner pipe:
The outer pipe may also leak at the head flange as well. The outer pipes were not designed to flow any exhaust, so there is no guarantee that they are gas tight from end to end.
I used this adapter on my '81/H:
This is a 1 1/2 X 1 3/8" adapter available at most auto supply stores, #548520. The small end will fit tightly around the outside of the inner pipe. Clamp the muffler around the large end. There is no way to clamp the small end around the inner pipe, but it fits real tight, so you can either wait for the carbon buildup to seal it or apply some Permatex muffler sealer.
The small end is completely hidden inside the header, the big end is completely hidden inside the muffler."
This is what it looks like when installed.
