Used Aftermarket Aluminum heads.

[ken-r-mer]

The way I understand it:
Vertical clearance over the valves at the backside (towards the intake): Assuming the angled valve at the high end (backside) is flush with the block deck, as I won't know till I remove the current head. The proposed Schneider P-3/8 cam has a 0.375" lift, then factor in a 0.055" compressed head gasket thickness. The valve then extends into the head a total of 0.320". The Abbin Tables 4-2 and 4-3 has the stock Weiand head pocket heights at 0.425" (exhaust) and 0.455" (intake). So a top clearance of 0.100" above the exhaust an an extra 0.030" more above the intake valve. I can see cleaning out clearance on the backside walls of the head, but not knowing the thickness of the ceiling material above the valves, I'd be hesitant to take away anything, other than to smooth out the surface...unless I had a scrap head to do a cutaway to know the thickness at the valves and the piston dome for sure.
At the heads and using a dial indicator, measuring the maximum piston rise above the deck and the depth below the deck at the piston edge (center to edge) for each piston, then with a cleaned head surface, measure the head depth at the center for each piston dome, then adding in the compressed head gasket thickness will tell the squish height. To get a uniform squish gap, having a custom cutter bit made from another same-make piston dome would be the only realistic solution for a LOCAL shop to do the work. Keep in mind changing head gasket thicknesses in the future changes that squish gap.

Now to get a uniform 0.040" gap between the piston and head while using a 0.055" compressed head gasket thickness, the piston edge needs to rise above the block deck 0.015". Is that correct?