When I built the split system for my C34 I was looking at using 2 Varex mufflers side by side, but the cost was going to be too much. So I was looking at what else is available and found Dynamax VT mufflers which for 2 was less than the price of 1 Varex.
http://www.dynomax.com/mufflers/dynomax-vt-mufflers
These use the same sort of thing, a butterfly valve but they are spring loaded. At light loads they remain close and under full noise they become near enough to straight through. They are nice and short in height and don't look any different from a standard muffler from the outside, which for me is plus. Once the car is warm, it's quieter than factory, but with a slight rumble to it. Full noise and it just screams.
Here's the only pic I have that you can see the valve in mine.
Lots of clearance when tucked up high.