Muffler gasket question

[autobonrun]

I have a Dinan dual out muffler connected to SSI exchangers. I plan to drop the muffler this winter to access a leak on the covers of my chain tensioners.

My question is whether muffler flanges are a universal size or do I have to find gaskets specific for my setup? What is a good set to buy that will form a tight seal? One of the SSI flanges was not perfectly flat when I first got it so a little thicker may be better.