Remember every time you order chassis, steering, underhood, etc parts for your 57 PONTIAC as far as the parts guys are concerned you own a 57 CHEVY and they will sell you ball joints that are correct.
A GM dealer can get you those under 89000801 or an auto parts store that carries Moog can order you a K670. They look to be pretty expensive though, over $100 each. There is likely some cheaper ones out there but the quality on the ones I mention is likely a lot better than the imported junk.
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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)
