you need a 66 part- the 65 had a one year only tank, and pretty sure the neck was different too. DVAP.com and allamericanclassics out west both have a lot of the 60's cars, but they arent cheap.
can you boil yours out and seal it to coat any pitting? ours had some rust in it, but was nothing compared to the tankfull of cinnamon looking powder...I did the chain/solvent, rinse with Ospho(phosphoric acid) rinse and coat with POR15 tank sealer, when I dumped it out brushed that on the outside too, gave it that galvanized look again
BTW, hate to sound like a broken record to anyone that comes in with a 65-up gal: have you taken a hammer to the frame yet? if not, you probably should- they can look fine and yet poke right thru with a thump... dont want to damp anyones enthusiasm but if its a problem, its one best found sooner than later...sadly it was a pretty widespread thing with the 65~71(at least) frames- as yours was in texas, inside, likely its downright beautiful underneath, but just wanted to bring it up...dont hurt to check.