Top quality stuff. I have a full system on my Cloverleaf from these people and am well pleased with it. They have lots of different designs you can choose from, be a damn sight cheaper than off the shelf stuff too.
I know Ansa 'did' make a back box for the 164 V6's but i dont think anyone stocks them now.
Sebring Exhausts used to do a stainless one, but you will have the same problem above.So that leaves Autodelta in London-they will have a stainless one one the shelf for you-but expect to pay around £250-300 for the back box plus fitting if you dont want to do it yourself. I had a system custom made from the middle to the back but at a different place to Mike and it cost me £150-but i did supply them with the back box and it was in mild steel and not stainless.