I assume you're building a chopper or a bobber with the stock frame. You will probably have to cut the 2 tubes and triangle off the backbone to make any of these tanks work.
These are all available at Lowbrow Customs
http://www.lowbrowcustoms.com/index.phpThe repop Wassel peanut tanks are more rounded. They have an angled tunnel to sit level on the frame.
![](http://www.lowbrowcustoms.com/images/products/detail_1626_p-nut-wassell-style-peanut-tank-bobber-chopper-photo-1.jpg)
Axed tanks are more rounded and have a mid tunnel.
![](http://www.lowbrowcustoms.com/images/products/detail_888_axed-chopper-gas-tank-photo-1.jpg)
Narrow Alien tanks are more oval and flatter than the Axed tank. They have a low tunnel.
![](http://www.lowbrowcustoms.com/images/products/detail_1213_narrow-alien-center-fill-gas-tank-photo-1.jpg)
If any of these fit the look you're going for, buy new. Don't mess around with old tanks that may be rusty inside or full of bondo under the paint. If you want something vintage that isn't currently repopped then you will have to deal with issues like rust, liners, and dents.
Scott