Seems to me you might as well just let the electric pump just keep running. It shouldn't interfere with the mechanical pump, the pressures won't be additive so they won't overpower your float valve.

Not sure why it was necessary on your boat? The main pump should come on with the ignition and pressurize the line on its own.

'69 1/2T stepside, Sea Foam Green, 250 L6 w/ 3 on the tree, not an option on it, in the family since new.