Ok back to heater gas tanks.....
I am thinking separate tank again. After talking with one diesel heater mfg they said they will soot up much less on kerosene at higher elevations. It would be handy to be able to do that.
Im thinking maybe this tank (early CJ 2a/3a) its 10.5 gallons.
In this space.
However, Its ~8.25" tall, not including the filler neck (im assuming!) And Ideally I would get it below the horizontal body line. That is the floor level in the truck. So I could cut in a small hole just below that to fill the tank. Tank is also ready for a sending unit, which could be a nice touch. Also I dont want it to hang down too far, as I dream of cutting the back end off at an angle one day. 10" or less is ideal.....
The other option is an RV basement/storage door in that location, and use a 6/6.5 gallon boat tank there, I would be able to build a cradle there pretty easy.