A little dirty pencil & napkin breakfast engineering:
Gravity feed gasoline will give a pressure of 0.3125 psi per vertical foot. An "A" with a full tank will deliver slightly less than 1 psi to the needle valve. The "A" float and valve will close against ONE psi (delivered over the surface area of a standard needle valve, .008 sq, in) when a 2" diameter hinged float with a 2:1 leverage advantage is submerged an additional 0.055" beyond its open free-float height.
If the pressure is increased the float will need to be further submerged to close the valve. A pump that delivers three psi will require the fuel level in the "A" float bowl to go .11" higher than the stock setting. A five psi delivery, requiring .22" further submersion, would likely overwhelm the stock float even if it were reset to a lower static closed position.
Beware of too much pressure. Measuring very low settings, like on a fuel pressure regulator, is inaccurate at best with a bourdon tube gauge. If you want 2 psi, it is best to use a Dwyer gauge or set up a simple manometer. Gasoline will rise .3125 Ft/ psi, so about 6 1/2 feet up a clear hose from the carb level is exactly 2 psi.
The later square-float "B" carbs had an assist spring on the float. Not necessarily a good idea because varying spring tolerances would affect actual bowl running level independent of the float height setting.
A key design point in a carb float/valve is to have as little fuel bowl height variation as possible with changing pressure. The difference between an A with a full and empty tank on the carb bowl fuel level is less than 1/16 inch. If you try to operate it on a pump that swings up/down several psi you will have problems. A calibrated regulator after a fines filter would be a good idea. Later design carbs use different float leverage ratios, geometry, and valve design to address the problem, making them easy to operate with a pump that does not have tight output pressure regulation.
Last consideration: If you have a "B" tank in the rear with a low mounted updraft carb and park facing downhill, a slightly seeping needle valve will cause a gasoline flood. You have eliminated the safety shut-off valve of the A. The B mechanical diaphragm pump would not freely pass fuel when not running. Many electric pumps WILL. Beware.
