6you said the engine was rebuilt. what parts are in the bottom end? if you are using cast pistons ans cast iron rings, dont go any bigger than a 100 shot. if you have forged pistons and good molly rings, you can run more.
ditto for the dual plane, you may want to look into milling the divider some just to give the n20/fuel more chance to mix with the air/fuel from the spray bar, but not to much or you will start to lose the low end benifit from the dual plane manifold.
also, what type of exhaust setup are you running? you will need a good exhaust to remove all the extra spent gasses made by the n20.