How much boost is needed? Intercooled? How much N2O HP? For low levels no changes. For medium levels you can go with some head work or just get a cam with the correct LSA and increased exhuast specs which is what most people do. Large amount you need a person who builds N20 engine for a living. Edit: Remember unless is a strip only car you are dealing with to different engine needs. With and without N2O. What will help the N2O can reduce the NA HP. I have never seen flow numbers where the entire motor was used. Normally the intakes or flowed using a radius entry but I have seen where the intake manifold was used for flow testing. On the exhaust side there or those that flow with no pipe and those that use a pipe. In most cases using the pipe will cause the exhaust numbers to be higher. If the person used a pipe you want to look at the diameter and length he used as this will change flow numbers as well. When flowed the head is bolted to an adapter which should have a diameter near the bore of the engine you are building. Again the bore size can change flow. Also check at what depression the heads were flowed normally in inches of water. Today most are flowed at either 25 or 28 but some use higher and older number were normally flowed at less depression. If you know what depression they were flowed at it can be converted to another depression.