Surely you could just setup a relay off the existing light sender. Pretty sure it pulls the light to ground so you should be able to wire the negative of the relay trigger to it without any interference to the light. You'll just have to wire the pump power off 87a as the relay will end up normally open.
Only problem I can foresee is starting. If its only say for a little while then there should be enough fuel in the bowl to get you started, build oil pressure and activate the pump. If the car sits for long enough to evaporate all fuel from the bowl then you would need some sort of bypass prime switch/button.
Alternatively, just run a t piece in the stock sender hole.