I don't know if you ever saw Spuddy's ( I think that was his name ) monster 5E on the old uksc site that was pushing 575bhp, he had the same problem with not being able to get enough fuel to the engine but whilst running nos. Iirc he got around the problem be replacing the dizzy with a big manual fuel pump and managed to get more then enough. Don't know if this helps but just thought it was worth a mention.