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

Well I believe he said that back in the 1970s when there weren't electric pumps available like today. In addition, I also believe the stock pump was intended to work as a regulator too.

Nowadays lots more better electric pumps, and not heating the gas from mech pump block contact, out ways any reason to run a mech pump on a race car. Though for a street car I just run a good stock type mech pump.