Built in water tanks need freeze proofing, or to be emptied in the winter.
How about a house lead acid battery bank where the spare tire normally goes. Lots of weight real low. I figured on a heavy duty rack to hold two or three group 31 batteries in that place.
Another possibility is a storage rack for jerry cans there. Two are easy to fit up there. Removing the spare tire hanging hardware allows more to be squeezed up there. The big issue is the exhaust pipe goes by there.
I decided that water would just be in LCI style jerry cans and keep them in purpose built lockers just in front of the rear wheels. Only works if you are making a back to replace the pickup bed. Depending on tire size, two should fit in front of the rear wheels on each side. Unfortunately it requires moving the pickup bed mount.

The mount needs turning around so it doesn't stick into the space. That gives 4 jerry cans for water or fuel in any mix. If you move the rear wheels back, more could be fit there. If you want propane for heating or cooking the propane tank could go on one side or the other and jerry cans on the other side. It is also a spot where the house lead acid batteries could be placed. Put one or two on each side in a lower locker with another locker above for other stuff. Note, the top of this locker is higher than the pickup bed floor. I figured that the wheel well hump in the back would just be extended forward. That actually helps make metal work for the back easier.
My current bed design has the interior floor in front and behind the wheel wells at the same height as the wheel wells. On both sides that space is lockers accessible from the outside. It would be possible to make them accessible form the inside, but I decided against that. It also means if I do lengthen the wheelbase later, I just have to rebuild the side lockers, while everything inside stays the same.
OK, steering geometry is best with the standard cab and 6.5' bed. That is to say it is optimized for that length of wheelbase. Longer and shorter wheelbase layouts have some built in scrubbing of one of the front tires when turning. Toyota gave all the truck length combinations the same front suspension setup. I've considered going with the extended cab drive shaft setup and pushing my rear wheels that much further back (11"?). I need to do a bit of frame repair to the frame rail tips. When I looked into the steering parts to find out what I needed to change for the added length, it was all the same part numbers. The reason I haven't done this yet is it changes the turning radius. Right now the truck turns around on a narrow road quite well. That is a feature I may want to retain for photography trips. I'll still add nearly a foot of overhang at the rear to allow a longer bed platform in back, but at this point I don't think I'll move the rear wheels.