The motor is a blown 383. I currently have a dual 3" system with an x-pipe and Ultraflo's. I don't really like the sound at high rpms. It sounds great at idle and low rpms, so I think its the x-pipe. It seems to quiet down and the tones seem to mush together. Open headers it screams and knocks all sorts of things over in the garage, sounds great just too loud. Cam is on a 110 lsa and 9 psi.

So was planning on doing a single 5" with a dynomax bullet, but dynomax is claiming that a single 3" muffler will support 800hp and 1800 cfm.

I don't want it to be SUPER loud, though, so not going overboard with the diameter is important.

Once I'm sure I'm doing a single, I want to figure out the correct collector length with a y-pipe.

I dont think you'll need to go that big. Dual 3" or single 4 should be plenty. There are guys with 1000hp with dual 3 or big single 4" setups.

I might put a hundred shot system on it next year.

Do you think I could get away with a single 3.5"?

What effect does the y-pipe have on collector length?

Y pipe will act just like one long piece of collector. I am not sure however if the length of the ypipe really matters. It should act sorta like a balance pipe in a dual system, since the pipes merge, pressure is equalized between the banks. I dont believe location of the balance pipe makes any difference in power from what I remember reading about exhaust tuning.

A single 3.5" you might beable to get away with but I think a 4" would be good idea just to be sure.

ive got a 4" single setup and its not killing a lick of power.
625-640hp here.