i just read a story about a guy building a slant and the head he put 1.88 intake valves and 1.50 ex valves in the head and also had it milled .100 and ported and polished the head.. so how big of valves can you put in a slant 6 head?
In a report about Steve Nitti's slant six Duster (supercharged,) it said that he had 2.02" intake valves and 1.5" exhaust valves. The report said that the cylinder walls had to be notched for clearance on the intakes even though the block was bored considerably... can't remember how much, but, it was a lot. Maybe, like .105"... dunno, exactly...
I think most people use either 318 valves or aftermarket 1.74" intakes and 1.5" exhausts. That's what my head has.