RHS BBC heads
This is not a direct answer, I know. But, I have used several sets of the PRO TOP LINE/ PRO ACTION/ RHS heads on BBC engines I have built without NOS. I have had no issues with them, they perform well and are a very nice, dense casting. The decks are fairly thick, the material is high quality, cast in New Zealand, in a up to date foundry. That said, why wouldn't they be good for nitrous?