I used old fashioned grinding on my oem manifolds which required an industrial-sized handheld grinder and an aggressive bit as they are made of some sort of Kryptonite alloy and are freakin' hard!
The surprising thing (to me) was the amount of core-shift there was from one end of the manifold to the other, considering not that many were produced.
Once that material was removed, a few of them had walls that were a little thin, but still safe and no longer blocked the exhaust port.
Did all that work pay off?
Probably, but just freeing up the restrictions blocking the exhaust ports made me "feel better"
Fitting the headers was the expected nightmare and a nylon ratchet strap worked its magic to pull the primaries in the right direction. One hole was off at least 50% (the primary was actually twisted) and had to be repaired with a high-speed grinder and a good bit as SS is also hard!
After the headers went through some hot and cold cycles, they were no longer under any stress and fit just fine.
Ceramic coating IS A MUST and the reduction in underhood heat is incredible.