I design and install commercial ductwork everyday. all I deal with is air flow (CFM, Velocity, Static Pressure) You could get an oval pipe to flow what that 3" round pipe flows. It's just easier to make it out of round and crush your fire wall. The stock ford pipe probably would be out flowed by a 2-1/2" piece of round tubing which would probably fit without any firewall mods. Does anyone make a downpipe kit out of 2-1/2" tubing?
Commercial duct work and PSD's downpipe (any turbo exhaust for that matter) couldn't be any more different.
Turbo exhaust leaves the turbo in a circular direction (think tornado). If it has to then pass through anything other than a circular pipe, the flow is disrupted greatly which equals back pressure which defeats the whole purpose.
That's why no one makes and/or sells an oval downpipe. That's why Ford also quit using a crushed downpipe like we got stuck with and went to as much as a 4" downpipe in the SD's - round. Not oval, not crushed, not rectangular... round.
A 2 1/2" downpipe?
To the OP. How much you have to bend the pinch weld, will depend on how the factory set your engine. If it's set forward you may not have to bend it at all. If it's set back, you may have to bend more than others. It's a crap shoot really.
Bending the pinch weld doesn't "require" a porta power. It just makes it easier. A sawzall, 2x4 and a prybar, and/or a BFH will take care of that bad boy.