does anyone know the current draw of the fuel pump supplied by mnr, to go with a r1 5vy engine, but i'd guess it's the same as with the vast majority of the injection engines

NO idea of the draw of the pump your talking about but mines an aeromotive monster and can draw up to 10A. Im guessing yours will be no more then this and probably somewhere in the region of 6-8A.

This figure can, (and does!), vary depending on the pressure set at the regulator.

Hope thats of some help!

i think that will be good enough to work on, i've got a limit of 18 amps, otherwise i'll need another relay to boost the power

Fuel pump circuit on MNR loom is rated at 10amps. This is more than adequate I have same setup as you and no problems with it.