I'm slogging through head replacement and have tried a technique that,
while I can't say it's worked out well because I haven't run the car,
seems worthy of mention for others contemplating the same procedure.

I had the heads cleaned, then dremeled out the corrosion to leave
bright, shiny pits and filled them with JB weld. Cured overnight. Then
I spray-glued 100 grit sandpaper to 3/4 inch smooth plywood, mounted
the sanding surface on a table, and sanded the head in a circular
motion until all was perfectly--and I do mean perfectly--flat.

I have pictures of procedure and results if anyone is interested.
Everyone may do this, I don't know. But it saved me a couple of trips
to the machine shop and went really quickly. I tried it wet and dry,
both have advantages/disadvantages. I'd just do dry next time.

Jim