Can you mount a HP fuel pump vertically (Outlet Downwards) as long as the inlet is still gravity fed?

Yes, doesnt matter what orientation its in

so you could point the pump upwards as long as the swirl pot is higher than the pump?

When I was mocking up the system for the MGB, which never got built, I was getting better results with the pump either horizontal or the outlet uppermost.

When I say better, the system primed up quicker, and gave a more consistent pressure.