I'm not sure what that bike is Sero, but it sure is dreamy!
I agree; the GS tank doesn't look much different from the side, although I bet from the top as you sit on the bike, the difference might be more noticeable. Still, if it's a Cafe look you are going for, something more square would be better in my opinion.
A GS-E tank of some sort is a great place to start; 450, or maybe even 750. The 1100 is probably way too big, but it too could look cool if you re-work the rear seat and such.
I think you've seen the 1969 tank that's going onto mine; Jay reworked and re-welded the frame to get it to fit, but I think he could have gotten it to work by hammering in the bottom of the tank as well, just chose to get crazy with it. It is a good bit longer than the stock tank...
No idea who built this little Savage roadster, but this photyo has been around here for a long time. Looks like a tank that came from Airtech; they have a bunch of cool designs, and they have all the measurements posted on the page to see what they have that might fit for just a few extra bucks....
OK, it's a LOT of extra bucks... but their site might give you some ideas just the same...
http://www.airtech-streamlining.com/vintage/vintagetanks.htm