I want to order a new gas tank for the 68 2+2 (396, 325 horse) and I'm a bit confused. I've seen all the other "gas tank" posts and they recommend Spectra Premium and usually the Chevrolet fullsize equivalent.
So, when looking it up through various website, it suggest the GM53A for Chevrolet Impala BUT on the Parts Avatar site it says this isn't correct for the Parisienne.
I know on other applications the interchange between Canadian Pontiacs and Chevy has been discussed and apparently the difference is a slight difference in the filler neck angle between Chev and Pontiac. I am betting the tank main body is identical and if it was me I'd likely buy the Chevy tank and try it. At worst you would have to remove the filler neck from your old tank and have it installed in the new one.
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1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars (now converted to a "factory" 4 speed)