Depends on whether or not your '73 was built before ~ November 1, 1972. If it was built before 11/1/72 then it is probably small ball joint and you will need ball joint adaptors or large ball joint UCA's. If it was built after 11/1/72 then it most likely has large ball joints and the 73+ disk spindles will bolt up to the UCA's. It's Ma Mopar we're talking about, so that date may have exceptions. If it's right around 11/1/72 it could be either one, but the further away from 11/1/72 you get the more likely it will be one or the other.
You can also check the actual drum brakes that you have. If it has 9" drums, you have a small ball joint. If it has 10" drums, then the small ball joint drums will be 10x2.25" and use smaller wheel bearings. If you have large ball joint drums the drums will be 10x2.5". It's in a factory TSB that can be seen at hamtramck-historical
https://www.hamtramck-historical.com/images/TSBs/1973/05-03-73C page1.jpg
https://www.hamtramck-historical.com/images/TSBs/1973/05-03-73C page2.jpg
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