Super Six install on 1974 Dart

opening the exhaust pipe wont help a bit, UNLESS you also open the outlet of the manifold/ to at least match your pipe size....
I ground out 2, /6 exhaust manifolds that came to me, (both with Super 6 intakes bolted to them) with a die grinder and various burrs.... both exhaust manifold outlets were well under 2-1/4", when I started.... the 2nd one I did, though the whole outlet hole in the manifold was within the confines of the ID hole of the flange gasket, I didn't have to grind the whole circle, probably ~3/4 of it, to get the entire outlet, out to 2-1/4 Diameter///
but the original 1-7/8" outlet hole "touched the side" of the 2-1/4" hole, in the flange gasket... 180* from where the outlet hole "touched" the flange gasket hole's edge, I had to grind ~5/16" of casting away to get that portion to also match up with the edge of the flange gasket hole, and tapered down each direction to where the manifold hole "touched" the gasket hole.... I bolted a flange gasket to the flange, and painted the ID to know where, and how much, to grind. That is the manifold that is going onto my /6. The 1st one I did// which is on my son's truck// wasn't entirely concentric with the flange gasket position to start with either, but it was much closer to being so, than mine. On my son's, to get to the 2-1/4 we wanted, I did have to make filings "all the way around" the manifold outlet flange hole.
hope this post makes sense, as I was pecking it out I had to stop a time or 2 and think... "wait a minute".....