I'm not trying to thread jack, but just curious as to why you'd go with a stock/used tank? Unless you have one lying around that you know is in good shape, or are trying to keep costs down, I would think an after-market tank would be a better option. Set it up specifically for the size and dimensions you want.
This is just an example of what could be done if you were on budget. I would be getting a stainless tank made to fill the whole rear of the frame where the spare resides now. Youre right though, finding a filler location that would work would be a PITA. This is all just an idea, not saying I would do it. Just wondering if anyone had a rear mounted tank on a Syclone that wasnt a fuel cell mounted in the bed.
Very cool. Only downside to this is moving weight from in-between the wheels to outside of the wheels. From a handling standpoint, this is usually a no-no (there are exceptions).