Well your guess is as good as mine...
On the GT40 by time I fit headers, I've got room for an 11" muffler and an inch of pipe either side. No idea how I'm going to fit my large ricer exhaust tips in yet.
So I've got an 11" muffler &a few inches of pipe after the collectors...I reckon it's going to be real quiet.
Seriously though, my brother drives one of the quietest cars in the club...silent, but deadly. Hairdryers make for very effective mufflers.