We used to put warped exhaust manifolds on the surface grinder--we charged 30 bucks a pair,and it took longer to set them up on the machine than to shave them flat again..
One thing though--the more metal you take off,the easier they'll warp again,and probably worse--also many manifolds are warped when not bolted to the heads,but will still flatten out enough when torqued down,not to leak..in other words,they may not need planing or grinding..
Also many small block exhaust manifolds "shrink" and make installing or removing the bolts difficult,and those spreader tools often end up cracking the manifold..if your are like that your better off buying new manifolds..
(I have put a manifold in my wood stove with a bed of coals and used welding gloves to install it--that expanded it enough to let the bolts align well !)..
I dislike using exhaust gaskets--GM never used any,and if you do,they'll usually blow out and create annoying ticking noises,the solid copper ones are about the only ones I'd use if I felt the manifolds would leak without any..
I think the plugs to block off those air tubes (flute pipes) for the AIR pump were 1/4" "straight" pipe thread,but tapered pipe plugs work--some years had metric flare fittings,those I just welded or brazed shut after cutting off the tube,that way they didn't have to be removed,and risk breaking the manifold..
