I suspect you may require About 400 cfm per engine
A simple calculation for cfm is to multiple the horsepower of your engine
Hence (2 x 150HP) x 2.5 = 750 cfm.
An 80 square inch of Free Area* louvre will deliver 400 cfm @ 720 fpm velocity**.
A 144 square inch of Free Area* louvre will deliver 750 cfm @ 750 fpm.
Dave Gerr determines the minimum vent area (in square inches) by dividing the engine horsepower by 3.3, then adding a 10-percent buffer.
Hence (300 HP ÷ 3.3) x 1.1 = 100 square inches
His smaller louvre, will have a higher velocity (750 cfm @ 1080 fpm), thus be a little noisier.
* If youy don't know the Free Area of your louvres, assume 50% of gross area, for estimation.
** 750 - 850 fpm are considered good "residential" velocities.