I had a rear mounted tank on my 56 PU and hated the issue of filling it with fuel. Eventually made an inlet that went through the chassis and came up inside the side of the bed.
That made the truck usable (fuel fill in the bed floor? Really?) but it spat back on fill ups and got really old really quickly....
So... think the rear tank is a possible non starter...
With that in mind then if I sump the in cab tank and then mount the pump to either the inside of the chassis rail (or possibly to a supplementary cross member along with pre and post filters) would that work?
Think the blow off valve 'might' drown out the pump....
P.
