pipe size from collector to tail pipe.

Hi,
Well I went with all the advice last time when talking about modifying my Tri-y headers. I scrapped the idea and went with the TTi 2" long tubes.
So if anyone is looking for a set of Tri-y headers I have some for sale.
Even with all the questions and order process they still required a fair amount of shimming and adjustments/beatings to get them to fit.
But now they are in with what I hope is enough clearance on everything.
So onto the pipes mufflers and tail pipes.
I have it mini-tubed and moved the springs in to the frame.
I cut a large hole in the trunk and sunk the 20 gallon fuel cell down inside to leave some trunk space.
I have read a bunch of threads with what seems like many different applications for different motors, hp, use etc.
So here is my info and any help would be great.
71 dart, 440 - 4 speed, 508 hp & 565 tq on dyno.
TTI 2" long tube headers with 3.5" collectors.
I will be running a FAST fuel injection with O2 sensor. Not sure if that changes anything.
As I said earlier, mini tub and spring relocation to frame with a fuel cell .
Thanks all.
:burnout: