I don't have a spare tire rack yet. I also have oversized tires so I need to find a heavy duty spare tire rack if anyone has any good leads on one I'd appreciate it. Don't like the idea of mounting the gas can inside either guess I'll be working on either getting a tire rack or making one.
I have had mine mounted inside, its not a big deal if you have good cans. You should vent them every once in awhile. The inside of my truck hardly ever smelled like fuel, but also no carpet or anything in the back to soak it up either. If it did have any smell it was no more then if you had a rag in the truck that had fuel on it at one point. I also only filled them 3/4 of the way full.
I moved them to a bumper style rack awhile back due to re-organizing the back of my truck. This bumper had a spare tire on it at one point.
They are easier to mess with on the outside for sure, however if your careful I think you could run them on the inside too without issues. The cans stay nicer on the inside and are less likely for theft.
The racks I have seen mount the cans on the inside of the rack to protect them from damage and theft. After swinging out the rack it allows acess for removal and filling, offset to the other side would be your tire mount also caged in for protection.