This is pure "heresay" but a guy in the next suburb has an 18RG running a huge downdraft of some description. The block and head were "o" ringed before the engine was fitted to a Corona. He has fed a 150hp shot of nitrous through it.
Went like a cut snake, so i've been told.
I would caution anyone against the use of nitrous, on any thing which has not been built for serious intrnal pressures!