On many motors a sealant is not used.-----On the 58 AD models there may not be a metal sealing ring.----If there is no sealing ring I would use a very thin smear of aviation sealant.----And the cylinder head is warped so make sure you refinish before installing.
Piece of sandpaper on a piece of glass or other flat surface such as a table saw.-------Rub , rub till surface is all the same.------You will find that the top and bottom contact first.----Rub till middle part contacts the paper too.
Thank you to all who commented. Took it out today and it ran great.
I think the issue was two of the bolt holes (one in the center and one just above it) were partially stripped. I drilled them out, tapped them and used helicoils. I replaced the bolts, as the two used in those holes were different from the rest. Longer and the thread didn't run as far up on the bolt. Someone obviously monkeyed with it in the past and half assed it.