What our our exhaust tip intake sizes? And what length should I get one? If mine is rusting up should I just get a new exhaust tip, or a whole muffler too (if it is rusting up too). What is the muffler intake/output sizes?

I want to buy a Windguard II for my 87 SE and it says they come in "Three universal sizes" (isn't that an oximoron?)... which of these "UNIVERSAL" sizes should I get? The 36.25 inch one, the 38.125 inch one, or 34.25 inch one?

Thanks,
-Chris