I was on a ZR2 S-10 forum several years ago wherein a gentleman put a 20 gallon boat tank in the bottom of his cross bed tool box and just used a pulse-pump and switch to refill his factory tank on the fly. Seemed to work fine for him. You could also do the XJ tank thing, and use the XJ in-tank pump to refill the main.
On my '96 GMC, I plumbed and wired in a switched dual tank valve that switched the feed and return lines, and also the fuel gauge. With just a little more wiring, you could also switch the pumps on and off, and then you'd just need to T them together with check valves and you'd have a true switchable dual tank setup with one gauge reading both tanks! That would be pretty cool!! I got the switching valve from JCW. You'll need the "6-way" valve to handle the returns.
You could also consider just carrying two jerry cans, one in each front corner of the box. That's 10 gallons that's there if you need it, and portable if someone else needs it.
Have fun!!
