I was lucky enough to get a virtually new rear muffler section with a parts car I bought. Since my middle muffler was falling apart (though not leaking yet) and the middle to rear joint had started loosening, I decided to get a good used middle section and clean it up and install it with my good rear section.
As it turned out, the middle piece I got was from a 20 valve car. It fit just about fine, except that the pipe to it was 60 mm OD, versus the 55 mm OD of the old piece (and the cat outlet pipe). I had to be creative to join them... Also the length seemed a little wrong, but then again, these exhaust bits never seem to fit "right" anyway. Luckily, my 1989 car had the appropriate "wide" heat sheild for this muffler. I used new chain type hangers for durability, and the usual stainless steel for the fasteners.
(I have since obtained a 20v catalyst assembly and attached its 60 mm outlet pipe to my system with a nice new OEM style stainless steel dual-layer coupling. I had to cut off the stock 20v front cat flange, since it was a four bolt arrangement, and weld on my best spare 3 bolt flange. This all worked very nicely.)
Oh, I also cleaned the parts, used rust-fix paint on what little corrosion was present, and applied many coats of this vey fine blood red high temperature spray paint before installing.