There are things one can do, port matching is a must. The last set of stock manifolds I tweaked for a friends build needed at least an 1/8" of material ground out of the port entrances on at least 2 surfaces to get it to match the gasket.

I have a set of reworked A body 383 manifolds that have had as much grinding as can be down withOUT cutting them in half and a piece welded in place where there was an internal divot to clear the flange bolt install. The divot was removed , welded from the outside and the flange tapped for a stud instead of the bolt. After all the work was done they were Jet Hot coated inside and out, I don't know if they were extrude honed. The manifolds were done by the late Greg Gessler.

I am going to try to compare them to an untouched set when I dyno to see how much it picks up.