According to the manufacturers of a low pressure electric pump, or at least the one that I used to have, it should be installed as close to the tank as possible and below the low fuel level. I looked all over when I did mine, could not find a low spot that was also safe, so ended up on the chassis above and slightly to the side of the differential. I wired it to "key on" to prime the carb, and it worked great. The only problem was that it that it was noisy.
I have no experience with using an electric as a "momentary" in line with a mechanical pump. You may experience vapor lock as the mechanical pump now has to suck the gas through the electric pump.
