I can't tell you for certain that you can get over 400 horse out of what you are doing but I think it should be in a reasonable range. You never said if you are upgrading the intake manifold and if the car is to be just street driven or ran harder. A single plane intake will help but you will give up some low end torque and driveability. I don't know what kind of gas you have access to but if you could raise the compression a little that would help some but not a huge amount in your case. Since you don't plan to go the stroker route, I would hazard a guess that the engine builder is pretty much in the ball park with his power estimates.
Ron
