Before you buy your keg, see if you can source a couple of "extra" M35A2 air tanks... plain janes, not the one for the split system. (iirc one of the split system tanks is an oddball for some reason, but I'll double check before saying for certain. )
Each air tank is a bit over 3gal, and can easily be plumbed for bottom drain, top fill, and end pressure. I'm looking for a cheap sealant for the tank inner wall. Those can be shoehorned into the pioneer rack/spare location, or right next to the air tanks with some fiddling.
Bigger than the 5gal Corney, and more correct for the truck and if you seal it right the water should be potable if you wish.
I'm low on truck cash myself. I've got a pile of parts and no time or money to buy more for a while. I'm knocking out the paint, the brakes, trans swap, then heater install, and the battery Equalizer, new seat covers.... ummm... then the EGT/water system? (That list is just the stuff I already have the parts for. )
Time for work,
Heath