Eliminating the cats will improve flow, sorry if you don't believe that, but you are right, the crimp is fail.

Yes, the stock downpipe is 3" (mostly). It might taper down towards the back a little.

 

It is 3" tubing the whole way. Except for the crimped part which is crushed down to approx 2.5" If you still have your stock DP measure it, it is 3" how many people are going to have to tell you that. I am looking at my stocker right now.

 

I used my stock flanges and flex pipe and welded in a mandrel bent 45* pipe I got from jegs along with an o2 bung. I only had to cut a couple inches off of the pipe to get it to work. Best 60 bucks ive spent yet on the car. But I have a welder and some good fabricating skills.