[vettestr]

Xavier,
I did see the Gage is reading after the regulator and before the carb so properly plumbed. If you have 12 PSI before starting engine the reg is not working right. I hear what Rob suggested and respectfully disagree. A regulator should not vary output pressure some 200+ percent between flow and no flow.
With a non bypass type regulator the flow is dead headed every time the float in the carb is at the top adjustment and flows again when needle and seat open again or a constant on and off cycle. Most stock needle and seats will be overcome by 12PSI and Edelbrocks web site mentioned 6PSI is what their street pumps flow. Regardless pressure should remain at set point on output at flow or no flow within a small % of change if working right.
A bypass type regulator flows regulated fuel at pressure to the carb and all additional flow is sent out the return port of the reg.
As a footnote to your wiring of the electric pump you mentioned it was through a toggle switch for on/off. As safety concern you should have a impact or G-force sensor in the line before the pump (available aftermarket or salvage on all newer injected cars). These often have a rollover function built in as additional CYA. If using a key to start engine I would also wire so key is in the ON position to enable the pump.