We are going threw this problem now on a 300 convincor with twin blown bbc's.
All i can say is screw Mercruiser on the Sea Pump/Fuel Pump deal.
Anyway we mounted two Barry Grant Hot Rod 220 pumps with return style regulators. The pumps are mounted under the back seat on the floor a couple inches from the fuel tank. The problem we have is keeping prime on the pumps. If the boat stays on the water there is no problem but if you trailer it where you are going up and down hills plus over bumps it will lose prime.
We can not mount the pumps level with the bottom of the gas tanks because we would have to remove the motors to even mount them. Its a cluster ****! I dont think you will have that issue since yours is a single.
If you are utilizing the Merc Sea Pump set-up you can use a aftermarket good mechanical pump BUT you will have to machine out the fuel pump lever arm out of an existing Merc Sea Pump/Fuel Pump and transfer it over to the the new aftermarket pump since Merc conviently decided to have there own pump arm design.
I did not know about doing this until after I spoke with a BG rep who was into boats as well. We have changed the pick ups around a little and still have problems with prime. We may pick up some mechanical pumps and modify them. I will say when we are up and running we do not have fuel issuses at all, it will run consistent 7lbs.
Maybe someone on here has been threw this as well and trade some more ideas. Have any questions just ask.