The external pumps - even good ones - can fail due to cavitation from "sucking" too hard on a stock pickup. An external pump can work with a large pickup but the guideline is don't go below 1/2 tank of gas. Its much better to go ahead and sump the stock tank, cut it for an in tank pump or buy a custom tank with pump up front. I'm considering this now after coming home from a road trip on a flat bed with a failed pump... not my favorite thing to do.

OE systems with an external high pressure pump like early/mid 80's BMW's used a low pressure pump in the tank to "feed" the HP pump. Bottom line, the HP pump does not like to suck.