A friend who was doing aftermarket exhaust systems in Ford Powerstrokes that had a flat downpipe used a hydraulic pry tool to fit a 4" downpipe in a spot where there was originally about 2" of space, but that was pretty drastic. That was done with the engine in place.
If you have enough room to swing that sledge, that works just fine. Back it up with another sledge or anvil on the top side and that seam will fold over nicely.
