I went to Sohan's in Trim. Contact them by email first and tell them the issues you're having. They are sound and can either tidy up what you have or (if you're flush) design a complete stainless steel custom job.
I just went for one of their stainless steel Magna-flow back boxes - TOP quality! My car is lowered (60 - 40) and hitting everything! they tidied up the rest of the system as it was'nt that old and still in good nick. The Magna-flow boxes are from the US, Sohan's are their Irish stockists. You can have 3 different types, they act as an expansion chamber as well as a silencer - it gave my car a lovely deep sound without being loud or "boy-racer'ish". Compared to the piece of restrictive rusty junk they took off the car the Magna-flow was a good buy. That was back in 2010 and the box is still perfect and the stainless is still shiny
Tell them you are a member of e30owners and you might get a discount