Carter is right, the NAPA muffler required flanges. I cut off my old flanges and MIG welded them on. The NAPA muffler has a lifetime warranty - for next time

5 years so far and still looking good.
From my email to MAXM37
The muffler specs (off the the muiffler) are as follows:
A<---->B
520308
9__109
I believe the NAPA 520308 is the part number. I welded the flanges off my old muffler onto the NAPA muffler as the new muffler did not have them. Very easy to do. If you run into a prob doing it, drop by and I'll weld them on (I'm off tomorrow). If you lack original flanges, bring or buy an exhaust gasket and match it up w/ the needed flanges at NAPA. Or, just cut a couple from 1/4" stock you have lying around if you need to save $10 bucks and have the time.