360 in 64 Val?

The bigger issue is not power loss but the flywheel. You have the smaller early A clutch, right? If the engine internally balanced? Is the crank drilled for a 4sp? The early A uses a smaller diameter clutch and flywheel with a different crank hub register, and it's not balanced for the factory type externally balanced 360. If the engine is internally balanced (would have had to be done when it was rebuilt) then you only need the bushing for the crank hub. If you can overcome that stuff - you'll lose a bit but overall it should still make more than what you have depending.