Hal,
I have a 351w 4v,I run both! My eletric is a carter "vane" type,running 6.5 psi.I have it run off a
oil pressure switch,straight to the battery/alternater. I have a stock mecanical pump also.I run a manual type gauge @ the fuel rail port,accuate until engine gets hot.The mechanical runs about 3.5 psi alone,with the eletric one off,(it will suck it through the non working pump).I normaly have both run @ the same time,and have 6.5 psi-pushing carb specs maximum for psi.Hope this helps.