6" high flow stack mufflers?

I've been looking all over the place, downloaded Donaldson and Walker catalogs, and I can't find 6" inlet/outlet stack mufflers anywhere. All the nice high flow ones are seeming like they're only available in 5". I live and park in a fairly quiet area so don't really want to resort to straights. This is on an 04 W-S 4900EX with the low max style pipes (6" from y pipe on, elbows ahead of fuel tanks, not the under cab exhaust that I see on most other non Lowmax trucks). I'm having lots of noise and dropping fuel mileage so it's really time to get them changed I think.

Where can I find a high flow/low restriction 6" stack muffler?

 

Well, it's got regular mufflers on it, and not spools or the chrome bullet type, so they've got to be out there somewhere. Mostly I didn't want to spend too much on something that's getting replaced anyway as I'm talking with a few people about revamping the entire exhaust from the turbo back.

 

Thanks! Somehow in all my googling Grand Rock never turned up. That's exactly what I was looking for.