New prop info
 

I have a 350 making about 400hp and it has a 19 pitch 4 blade revolution 4 that goes around 4200 rpms with also lower drive gears.I now installed a BM 174 blower that makes around 5-6psi.I want to be around 4800 rpms total,so what pitch prop am I looking at here in three or 4 blade? Also why don't I have a good top speed with this amount of hp on a 5000 pd boat, only goes 40mph?

forget the horsepower, a 19" prop turning 4700rpm (trimmed up, running clean) should be at approx. 50mph.....you better check your speedo for accuracy.....
A long time ago, my Formula (also about 5000#), at 4700 rpm, 3 blade 19" prop = 52 mph on speedo.... compare, then fix it !! may just be a plugged or kinked pressure line. good luck.

What kind/year boat??? Did you have 400HP without the blower??? If so the Blower should push you over 500HP....I hope your running a Bravo with that small block setup.