I have 90 gallon auxiliary fuel tanks on most of my trucks what I do for simplicity on my 94s I have 38 gallon rear tank (Bronco style) and I take the filler neck and weld in a 3/8 steel pipe nipple a fuel inches from the top or the fuel cap so I can see if fuel is flowing. The using 3/8 steel hardware and a high quality ball valve installed in the auxiliary fuel tank drain, then I ran 3/8 low pressure hydraulic line ( I use it for all fuel lines, good stuff).
It sucks when you walk to your truck the next morning and find a big puddle of $5 diesel fuel on the ground, because you forgot to shut the fuel valve off
. To avoid over filling I installed an electric shut off solenoid (normally closed) then I wired the solenoid to a toggle switch that is hooked into a key on fuse so fuel transfer can only happen when the key is on.
On my 00 F550 and 99 F450 I plumed the fuel fill line into the Ό auxiliary fuel port on the factory 40 gallon after axel tank. I think this is and option on the cab and chassie trucks to run a diesel generator, compressor, welder, ect..