If you buy the 38 gallon bronco tank like shown in the first link above you can re-use your factory pickup/sending unit. You just have to modify it to be longer (the tank comes with the necessary hardware to extend it) and make some new slots for the tabs to hold it straight.
I did the 38 rear tank and now I can hold 60 gallons under the truck!
i plan on doing this mod myself when rebuilding my truck. Jim's write-up is quite clean, but is there really a need to extend the return line? i always like to dispense with extra work where possible :-)
I think you would get some air bubbles forming while the fuel drops I to the last half of the tank. Also you may get siphon issues if the return is left out of fuel
The hardest part of modding the sending unit is the insertion of the extended rod thru the plastic part is keeping everything lined up. My original sending unit was falling apart and when I tried it, it fell apart. Believe you me, a new one is not cheap and I sweated getting the rod thru.