Pre-Liberty EFI Subarus had an external high flow pump (Leone, Vortex, + VL commodores, XF Falcons)
If you're only after a lift pump for your surge tank, pretty much any external electric fuel pump to suit a carby engine should work. They are self priming & if they've only got to fill the surge tank should supply enough flow.
when does your EFI pump turn on?
ie does it start pump when you turn the ignition to "on" or when you turn the ignition to "start".
If it comes on with "start", then wire up the pre-pump the pre-pump with the ignition "on".
When you go to start the car, turn the ignition to "on" for a couple of seconds to make sure the surge tank is "primed".