You buy a pair of 993 HE's you need to slightly modify one, the right one needs the flanges flipping because there opasite on that one to the 3.2 easily done. You also need to re route the oil line for the early type which is about £40. Then they bolt right up. I got some 993 bischof HE's on fle-bay for £31 yesturday. Then you buy MK's modified 2 in x out muffler for 3.6 conversions and which from the price ben sent me today is only $25 more expensive than the STD muffler. Much cheeper than SSI's and probably as good if not better.
If you wanted to do this I bet you could get your money back on that muffler selling it here in the UK anyway. So many people are still weary of buying from abroad and are much happier to pay a little more for something in there own country. Get it on impact bumpers
That spotless back date of yours deserves it
Steve