A factory Slant 6 with a 8 1/4 differential

My duster does have factory disc in front. But heres another mystery my dad had a 74 duster with a stock 318 and front disc he told me his came with 7 1/4.
I also have 2 copy of the build sheet one destoryed and one in perfect condition. I'm in the middle of decoding the paper work.