The first problem is he has long standard supply wiring to the pump and a larger pump. The Walbro draws about 15 amps, depending on pressure demand, the particular pump he gets, and the pump voltage.
The second problem is he is getting bad advice. There is no reason at all to measure voltage drops with the pump off. Measuring with the pump off is useless.
The wiring for the pump runs through the trunk stuff, forward through harnesses and connectors, through a relay, and eventually through the engine compartment harness up to a start relay fuse link. That 14 ga path COMMONLY has about 1/2 volt drop with a stock pump. That drop is perfectly fine with a stock pump. He now is drawing nearly triple the current. With triple current, he can expect triple voltage drop, and that is about what he measured.
That tells me his wiring is normal because the 1.4 volts drop he sees under load is reasonably expected. If he had a bad plug or some other issue he would have a heck of lot more than 1.4 volts drop. It would also be overheating the pin and would not last long. Since no plugs are melting, nothing has caught fire, and the drop is pretty much normal his choice is to pull a FUSED number 10 or 12 from the start relay battery and alternator common back to the trunk where a second relay is mounted, or to live with 13 volts run voltage on the pump. The 13 volts he measured is actually fine unless he has no headroom at all on the pump flow. If he needs 55 gallons an hour he will be in trouble.
His system is normal. If the 1.4 volts bothers him, then he needs a dedicated direct 12-14 ga fused wire and a trunk relay with the coil triggered from his OEM system.
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89 LX. 363, single turbo, Super Vic EFI, TFS high port heads by TEA, solid roller, glide. Holley HP EFI. (exact combo varies)