Fuel Tank Question

[MUDKICKR]

I recently received a 30 gallon square tank that would fit perfect where my rear tank is. i could make my sending unit fit in it, and i can also make my filler tube work, thats not the problem, but what i want to do is tie it into the frt tank and only use 1 sending unit. after i get the bed on my truck the way i want it im going to build a 3rd fuel tank to hold at least 100 gallons, but id like to set it up to where i use the factory switch to switch between tanks and use a factory sending unit in the 3rd tank. what do you guys think would be a bulletproof system to do this? welding on a fuel tank is no problem for me at all.
would just welding a fitting in the bottom of both tanks and teeing them together be about the only way to do this? or should i build a sump into both tanks? i know i will have to leave the return going into the top.