Where would you move the exhaust to?
Noting, of course, that the exhaust doesn't stay cool for long, so it needs to have some considerable distance between it and the highly flammable contents of the fuel tank.
I'm not sure there are many options available there. I don't think I'd like the exhaust to end under the cab, and I doubt stacks would fit although you could make a few rednecks* excited by fitting working stacks, 12 spotlights, 4 antennae and metre-wide mudflaps on the vehicle. Don't forget the "Bundy Lives Here" sticker across the tailgate.
One thing you could do is replace the rear bumper with some barwork that includes two tyre holders (these are available as after-market items) then use the spare wheel area for a fuel tank. The standard wheel occupies about 116.7 litres so you ought to be able to get a tank in that (with wall thickness, baffling etc) ought to hold about 110 litres.
* My apologies to anyone that actually drives a vehicle like that. The term "redneck" was intended to be humourous, not offensive.