From what I have seen you wont be able to skim much, if anything, off the head before a valve kisses a piston, unles you cut bigger reliefs for them. The M42 has knock sensors and unless you are going forced induction the oil squirters shouldnt be necessary, why not just drop the M42 straight in? I have match ported and polished mine, there is a bit of room for improvement there, but the cams will make the biggest difference. I also made custom headers as the factory ones are a bit too small for my liking, and they always crack at number 4 pipe where it meets the flange. The 6mm /7mm valve stems diameters dont seem to make too much difference, but the 6mm ones leave a tiny bit more room for porting. Also, the valves are sodium filled so dont cut them in half! (Sodium can explode on contact with water)
Good luck