The in-tank pump is a small centrifugal pump, much like the water pump or a "squirrel cage", and it will have no problem passing liquid directly through it without power.

Here's a suggestion; use the original pump to feed the carbs directly. It should have both the pressure and more than enough volume for your needs. If you're not comfortable with that, install a "tee" on the suction side of the mechanical pump and a small restrictor leading back to the return line. The mechanial pump won't have to work as hard by delivering fuel right to its inlet.