"you can build more pressure into a pump but i haven't figured out how to take pressure out of a pump." It is easy, you increase the area under the diaphragm. This allows a lower pressure to resist the force of the diaphragm spring, while increasing volume of fuel pushed during each stroke.

It is a system, everything from the pump, and lines to the carb seat diameter and "float size" matter. That is what most folks don't realize. Orig carters used a larger float in the AFB. Edelbrock used a smaller float which means it can handle less force to maintain the same float level. That is why edelbrock has a lower fuel pressure limit.

I have posted this picture before. The 6903/4862 use all the same parts except diaphragm spring. It has a small outlet chamber of about 26cc. The scalloped chamber is the hemi fuel pump. It holds 32.5cc and has a greater surface area. Other pumps like marine pumps used this larger hemi chamber.

Pressure is a measurement of the restriction to flow. It is a balancing act. Higher pressure at inlet can also mean less flow out of pump because the pressure resists the full stroke of the diaphragm and it short strokes and hangs until the lever comes back up and pulls it up again. Of course, too low a pressure can also mean your pump is not sized right and insufficient volume to keep level in bowl high enough to start restricting fuel flow.

So you can take your 6903 and machine out some scallops in the bowl to mimic the hemi pump, increase area and volume under diaphragm, to reduce the maximum static pressure out of the pump while gaining more volume per stroke.

Maximum pressure in the system is when the spring is compressed at top of stroke because flow is at zero. The further the stroke moves off top dead center the more the spring unwinds reducing the force on the diaphragm which lowers the static pressure when diaphragm/flow stops (because needles closed).

So while you repurposed the vapor separator to manage fuel pressure it was never designed for that and you reduced the flow rate of the pump. If it is over sized for you combo, no lose in performance. But it does lower pressure and reduce available flow rate. You can do it just as easily with a throttle needle valve add as a return line. But as you lower pressure you lower flow "rate" too even in a constant displacement pump. Because the speed the diaphragm moves it not mechanically connected to the lever during the output stroke. It is spring force against system restriction, with force decreasing as the stroke increases moving towards the bottom.