i have a similar early model. I bought a stainless "straight thru" rear box for it, used from ebay. Someone presumably had it made at a local exhaust shop. It was just a piece of 2.5" stainless pipe bent up to fit the bottom of the car. I wanted a silencer, and happened to have a small one from a BMW knocking around, so i cut the pipe, welded in the small box, and then instead of the streight outlet, i tacked on a polished stainless 45 degree turn-down tailpipe from an Audi diesel.
Looks neat and tidy and suitably OEM without being shouty. I'll snap a photo tomorrow for you if you like.
I guess the take-away is, find a local exhaust fabricator and get them to make one. I've had a few systems made (my local guy made the mid section for the rangey after it rotted away) and actually, its more or less just as cheap as buying from DoubleS, and you dont have to crawl underneath it.