Steering Box Difference '72-'73
I do not think there is much difference at all. I converted my 1973 dart sport (basically a dodge version of a duster) to manual steering by installing a manual steering box from a 1970 GTX (B body) and using a manual to power steering adapter coupling from mancini racing.
So if you have access to a manual steering box and steering column from a 1972 duster, it will fit in a 1973 duster.
if you just change the steering box you will need one of those manual to power steering adapters (for about $125) and you are in business.
hope this helps,
Bob