GQ Sub tank pump

[Patroler]

Just picked up a LRA Aux tank for the GQ, i want to run a pump (facet or similar) to transfer to the main tank when it gets down to empty (should be able to hold the whole 73L sub when the mains reading empty - still has 10-15L in it)
Just wondering if anybody has se up something like this so that the pump turns off automatically when empty?
Was thinking i could use the facet pump (it runs around 7psi) pump through a filter into the main tank, on the line between the pump and filter I could tee in a 1psi Hobbs switch (NO) and wire it to the pump (via a relay)
That way the procedure would be is: turn the pump on, hold a prime button for a few seconds (which would force the pump to run as the pressure switch contacts would be open) when pressure builds (due to the filters restriction) the contacts should close and hold the pump on until the tank runs out, at which point the pressure will drop shutting off the pump!

Anyone see any problems with running it this way?
I'd really prefer the transfer pump over a 6 port valve as being a diesel, i could totally run the sub tank dry - do that with the valve and i'll have to bleed the fuel system.

Also how have people with GQs and two tanks hooked up the fillers, i could turn the original neck into a Y neck - would a hi flow nozzle fit into both if i do this
Or Have two caps under the fuel flap
How do GU's fill? is there filler neck interchangeable with a GQ?
Thanks for any answers

Mick