[D13]

Moving the tank out of the cab was a huge leap forward in safety, smells, etc All the write ups you will find are how to get it out of the cab.

Old generation tanks won't fit directly. It won't fit well at all unless you limit the rearward seat location. By time you are done with the fab and proper sealing, you could have had a proper tank made, inside the rails.

If you have an electric rear winch, or unless you are running a 10 ton PTO driven one, there should be room between the winch and axle for a pretty decent tank.
Only reason I can think of to run exhaust outside the rails is a wiled motor swap or running stacks by the cab, since there isn't room to sneak the pipes past the tires. Then the area behind the rear wheels, or between the stacks and the wheels, should be available. I bet a set of 16 gallon tanks would sneak in there.... and the fillers would be accesible from the side.