How much can valve head OD be cut?

Cut any stainless valve down you want.

What's the diff between a 1.88 &1.94, .060?

Reading a lot of parroting portrayed as 1st hand experience. Seeing my own numbers and things awfully close gives a funny notion...
None the less... I have run 184 and 188psi with .030 quench and iron heads.
Aluminum heads will not make it detonation proof or any substantial psi ability than iron heads.