You can either drill a hole in the sending unit and braze in a nipple to hook a return line to, or buy a new sending unit that already has the extra port. I think '70-up had it for emissions purposes.
Long before the Inertia switches were available I used an oil pressure switch. I believe I have this one on my Valiant: http://ecatalog.smpcorp.com/STD/#/vehicles/parts/num/PS-64?type=s In any case there's a lot of options there too. I don't think that I'd bother with both, but one or the other for sure.
Put them in the trigger circuit of the relay, not in the switched power side of the relay. Works the same and there's no need to run pump power thru either switch.
Based on an estimate of the total circuit length and the blue pump's amp draw, 12 gage wire is the smallest that should be considered for the pump power wire.