I used to do this, but there are some issues. You of course should mount the electric near the tank. However, with the mechanical only, the restriction through the non-operating electric will cause a drop in the suction line, and could CAUSE vapor lock. You might be better off just running both all the time.

Your GTX is a '70? If so, you should already have the necessary vent lines to hook up a "vapor return" system like my six-pack had. I'm a HUGE believer in these. I don't remember the part no. but Wix (NAPA) should still make "universal" filters with a vapor return fitting. You simply mount the filter with the vapor return "up" and plumb the small line to a return line back to the tank.

If your car came with a tank carbon canister system, you should have a line already under there that you can "tap" into.

Also check for ANYTHING that can restrict the suction side, which will aggrevate the vapor lock condition.

Is the tank/ cap properly vented?

Are the suction hoses in good shape, and not collapsing?

Any chance the feedline is rusted, leaky?

How about the suction screen in the tank?

You should test the pump for proper suction (vacuum), pressure, and volume. Vacuum/ pressure test gauges just aren't all that expensive at any parts store.