For the RPM range, typically with forced induction, the usable powerband is brought down lower than in a N/A setup for the same engine. So you might actually end up with a peak between 2500-3500 RPM. So 4000 RPM might be a realistic top out for your revs. Going any higher is just unnecessary wear and tear. Plus being a Flathead design, it will probably still be a poor breather even with porting and bigger valves, but still better than if you didn't do those mods.
