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Just took a look at car-part.com found several listings as low as $25.00 each.
Do a search there for 1973, then F350 (Not Superduty)97down, and you'll see them listed, I'd suggest calling whomever you are interested in getting rims from, as many of these ads are a little old, and the folks don't seem real interested in returning Emails.
Pretty sure they didn't have 16.5s after 79. The only vehicles, that I know, to come with those wheels was the rears on the 73-79 Camper special trucks with the long wheelbase and the spare tucked up in the right side of the bed.