Larry Shepherd’ Mopar Small Block Engines book says keep mech fuel pump when adding electric.

Partially true. Many do not understand how a mech pump works. the arm does not pump gas, it rather, cocks the arm on the spring. The spring is what pumps the fuel. When the thing pumps up to pressure on a closed needle and seat AKA at idle, the arm floats on the cam for a stroke - or - a few. So the spring determines pressure.

But if the electric is already putting out more pressure than than the mech pump, then the electric will be the determining pressure. The electric will just blow fuel right through