pipe size from collector to tail pipe.

Thanks,
I looked up the pipes that are required to weld my own, and it is probably more expensive to do it that way. With stainless pipe you have to purge the inside of the pipe in order to keep a good weld without contamination on the inside which is where it will rust like mild steel.
The argon to keep the purge is around 80$ a bottle from Prax's air, or TSC.
I am not sure how many it may take to keep the weld clean.
I guess if I knew how to do it and wasn't mocking up every joint each time it may be worth it.
I may try to get away with turn down's in front of the axle for now and make awesome pipes up this winter.
Keep the suggestions coming.
Thanks all.