Gen III go here
http://www.quantaproducts.com/ they give measurements for most tanks, the cuda /challenger for `70 /71 fits, although mopar uses 240 ohm senders, if using aftermarket gauges its not a problem, just order the gas gage for a mopar, if using stock gauges you`ll have to convert the gauge or sender. the tank fill is separate, which is a plus if you need to adjust where you want it to come out, you can cut/weld to your liking or order one from another body style mopar.