2 problems, they never made a 6 cylinder road runner (that I know of) and the challenger was never a fastback or made b4 1970. You may have just got the models mixed up. :)
I stand corrected the fast back is a charger I own one that I bought from the yard as far as the 6 cyl. road runner the vin numbers were verified by Chrysler according to the owner of the yard.
