Brake conversion 73 duster

73-76 A bodies with 318's could also have drums on all 4 corners, and small bolt pattern.

Disk brakes were standard on all 318's starting in '73. They were even standard for a /6 if you ordered a sure grip. Right in the dealer manual. Hamtramck-historical has the Dealership data books posted, these are links and excerpts from the Dodge book but it's the same in the Plymouth book as well. For all the '73 up cars. In mid-'76 all of them went BBP disks due to federal safety laws.

https://www.hamtramck-historical.com/images/dealerships/DealershipDataBook/1973/73_Dart0014.jpg
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https://www.hamtramck-historical.com/images/dealerships/DealershipDataBook/1973/73_Dart0019.jpg
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Never seen a factory V8 without disc brakes.

At least on a '73+ car it would have needed to have been a special order, if it was available at all. It's Ma Mopar so I'm not sure I'd say "never". But drag race guys used to swap out the disks for drums, especially early on. They thought the rolling resistance from the pad knock back on the disks would slow them down.