I saw this for sale in Elkhart, IN today. 1981 F350 Ranger XLT, standard cab, dually. 460, 4 speed. I think I've seen one other early standard cab dually years ago.
Shows 97XXX, but it's probably 197XXX, or more. Body was average for the year, had van bucket seats in it.

 

They're out there, but most of them were beat to death.

 

Not rare per say, none of these trucks have attained that status.

However, they are slightly uncommon in certain areas, especially down here in Florida. There are probably plenty of them still floating around out west.

As for the motor, it shouldn't have a 460. Should be a 400 if it's original. But it could have been swapped at some point in the last 33 years.