Recently completed an install on a 90 and apparently there is no low side output support for the fuel pump; thus pump is constantly running with key on by grounding the FP wire at diagnostic box. I would like to set up a switch on the dash so that I at least have the ability to turn it off without getting under the hood. My question is whether I should use a relay along with the switch and if I should use a specific amp switch.
Just use one of the available GPIO positions and then configure it as a fuel pump. You can configure prime and any other attribute you'd want for a fuel pump. It's obviously going to be a low side output requiring a relay.
Depending on the fuel pump you might want to consider a pretty hefty relay if it's an OEM pump or a lower current pump you can utilize the factory relay.