...decided to upgrade the rear main seal with a 1-piece unit from FastFish, and while I've been waiting for it to arrive I put some elbow grease into 'machining' the mating surfaces of a pair of good used exhaust manifolds I scored from 440Source. I used a nice flat 1-meter level with a 1.5" wide base, some double-stick foam tape, and some 36 grit sand paper I had lying around from doing body work years ago...and just slid that thing back and forth...a LOT...until both surfaces were clean - ze-ro gap. One of the manifolds had about a 1/32" gap between the outers and the middle flange - the middle had warped 'away' from the head surface...and once bolted down would have contributed to cracking the center after a while, but now that possibility should be all but eliminated. Worked good - and I can put the $180 it would have cost me to have them machined at the engine shop back into the build. I'll try to resurrect my blasting cabinete and clean them up, then hit 'em with cast iron manifold coating.