I picked this up on the Forum - Chuck Brandt's build site
http://www.chuckbrandt.com/
Towards the end of his build he replaced the ERA undercar exhaust with one of his own configuration and exited it in front of the rear wheels. Lots of good info and photos of it.
I want something larger than 2 inch undercar exhaust on my car also. ERA was pretty noncommital in trying to increase it so I just ordered the headers and I'll see what my local custom exhaust shop can come up with once we see what clearance the rear tires allow.
Dan