There are other considerations as well. The sporty tanks tend to have a very narrow tunnel for the frame backbone area. There would be no room to put rubber cushions in there even if you wanted to, for most applications. I think the newest ones, from 2005 on are a bit wider in the tunnel but I may be wrong. My 2006 seems wider than my 1989 did. It may just be remembering it wrong though. It really would be better to mount it with the current bolts up front. Depending on what tunnel clearance you need, considering your coils are right there too on that bike, you'll need to relocate those as well, and maybe some of the wiring/running of cables too. Also, depending on what year of tank you go for, they bolt on at the seat differently and they are a different length between front/back over the years. That is the place you can at least save some headache on. Pick one with a setup/length as close to yours now.
The peanut tanks are ridiculously small for any real saddle time without refueling. I'd go with at least the bigger custom size of 4.5 gal or at the very least the 3.5. Looking at my sporty next to the 650 in the garage, I think the peanut tank would look too undersized. My 3.5 is as small as I would go personally. Oh, and the ones starting in 2007 were fuel injected so there are some changed from that as well. Then some have vacuum lines and.... etc.
Stay away from the AMF years. They just didn't have the best quality on anything back then.
Good luck and I look forward to seeing what you do with it. If you decide to change tanks, please think of us on the forum if you decide to sell your current tank. They are hard to come by and yours was specific to that year only. It was different all together in '79 and it changed in '81 just enough to make it inconvenient in some ways.
