I'm looking at adding an electric fuel pump to my carb 454mag. After I install the electric pump, should the mechanical pump be removed from the sea water pump assembly or can I leave it on there in case I should need to go back to it? Or will it be damaged without any fuel running through the mechanical pump?

it will be damaged,due to dead heading. remove it and block off the opening with a block off plate. if using an electric make sure you use a oil pressure hop switch on the ground side of the electrical circuit for the pump, and use relay to power the pump as to cut down on any excessively high draws in the bilge.

i run both in parallel, so if one pukes im still good to go home

I wouldn't bother swapping to a electric. It's been prooven on this board that electric pumps don't survive in a marine application.

I was wondering why you would want to switch ?
I have Electric Back-ups on a Press. Switch but woulodnt want them instead of a Mech

Run them both,,, Put a tee in the line before the Mech pump and then put a Reg before the Carb ,,, You will have the best of both worlds and not have to remove the Mech pump,,, Like said be for ,,,, run the Elec of the Oil pressure ,, or ,, I have seen them hooked up to the Tach ,,, so they come on at 2500 rpm,, or when ever you want..

RHC

Do it the way RHC said to but use a MSD rpm activated switch with a adjustable low rpm module.

Don't need it....why do it?

ditto!!!!

My boat came with electrics and mechanicals, in parallel. I have not had to use the electrics at all. 750 hp each @ 5500rpm. Never saw a drop in fuel pressure since I've had it.