I built my own home-brew 4" exhaust, and it was a COMPLETE PITAzz....
does cool better tho....we air sawed out the firewall, and installed the pipe, then built a steel cap for the firewall....
I used a standard short 90 degree bend from NAPA, and a long 45 degree bend I got from a truck shop.....used a slip-together joint in the middle,
about where the 3" aftermarket pipes do, witha 4" stainless band clamp. I'm still using a stock style turbo, so had to fab the end to go from 4" down to 3" at
the turbo. cut the end of the 3" downppipe I had on there off and used it over....and fabbed a plate that was 4" in diameter, with a 3" hole slightly offset in it......
mocked it up and tacked it. works very well.
it was not fun..... I suggest that if you are wanting to do this, do a 2" body lift first ....