Grant,
Take a careful look at the 6700 flats. Some were made from 30 ft stock cars as well. It is very easy to look at the flat and see it orgin.
From memory (that means all bets are off) the 6700 to 6724 were stock car converstions
From 6725 up were box cars. That may not be the right cut off number but the point is sound
Kevin