""Why are you considering adding round mufflers under the floor boards? If you need the extra sound reduction, wouldn't an oval give you better ground clearance, and allow a bigger volume muffler to be placed there?""

Dave,
I agree with what you are saying about the oval mufflers, but my thinking was that I would not find an oval muffler that was thin enough, so that is why I was guessing a 4" round muffler.
But, I found an oval muffler made by Magnaflow (#11226) that is 4" thick which is also a straight thru type of muffler (offset/center). I may still need to add round 6"-7" mufflers beside the tank to keep the noise down, but the oval straight thru muffler should help allot on noise reduction while keeping an open exhaust.

I may end up with a setup close to what you have, except both mufflers will be the Magnaflow's and 2.5" pipe.

Note: The above MF muffler will fit the TTi non-muffler design with one minor change. The OA length of this muffler is 2" longer than the Super Turbo part #17733 that TTi references. I would just have to cut off 2" off of the X-pipe length at the muffler end.

Ref: Dynomax: 2.5" #17733 (TTi option) 4-1/4" x 9-3/4" x 14" body. Per Dynomax: 410cfm (186hp)
Magnaflow: 2.5" #11226 4" x 9" x 14" body. Per MF: 530cfm (232hp) per muffler. (these are unrestricted numbers per the manufacturer)
2.5" exhaust pipe: 509cfm

Thanks again for your help Dave.

It appears that I will be looking at an oval and a round straight thru muffler per side. This setup will give me a 2.5" "straight thru" exhaust setup that includes a x-pipe (TTi non-muffler exhaust kit) and 2.5" stainless R/T tips (Accurate). This should be as quiet as it will get for a 2.5" straight thru exhaust setup.