There's a good bit of room to the left RIGHT rear outboard of the frame but the rear AC lines intrude a bit. Something like a medium-sized scuba tank would fit there, with some sturdy mounting brackets. Maybe no more than 5-7gal tank at 'normal' air pressures. Was just poking about under there the last couple weeks, with pulling the full tank and changing the rear coils and lubing all the rear suspension bushings. I'm thinking to put a tank there myself, a compressor where my rear sub is (rearward of your ARB install) and hard-plumbing QD couplers at both front and rear bumpers. I'd like to put a tank in too. And that rear quarter spot looks like a likely candidate.