For what it's worth, mine neck's down to less then 4" as it passes the torsion bar. So far with that tied to another 8 feet of exhaust and muffler it's supported 850+hp on single exhaust.

So as long as you're not trying to make more then 1700hp (850hp x 2) you've got plenty of room in the backpressure department to downsize.

I suggest getting the hole in the fender where it belongs, and finding a way to connect the dots, even if that means reduced diameter or pie cutting something to get the bend tighter.

Don't give up!!!