I plan to go EFI with my 70 but haven't decided whether to go with FAST system or MSD Atomic.

I'd run a separate FAST pump for the aft tank... not simply a lift or "priming" pump. Its not the pressure that would be my concern... it's the volume.

Return lines.... I don't think the tee will work correctly and force feeding either tank could result in overfilling... again, it's the volume of fuel. I suggest a fuel solenoid valve that switches the return line to feed the tank that is being used.