1970 Duster base model slant 6 rear differential 2070051 spider kit

I built a 1933 Dodge in the '80's & it has made it back to me for repairs on the rear end . Both spider gears are wrung off. This is the rear diff. considered to be junk by many but it fits in width perfectly. It is about 57.5" drum to drum. It is the 9 bolt 7.25 rear end. I bought a C7.25BI kit but 1 seller said I have to use an '80 model case for this kit to work. I don't know much about rear differ. but the seller said the case she is talking about changing is the case the pinion gear bolts to. Will the kit I bought fit the rear I have or do I have to change the case as well? If so what year, make, model cases will fit my casting #2070051? Could I use used spider gears & if so can you give me a source? Can anyone help me out here? I am in a real bind. A certified mechanic is coming tomorrow to see if the kit will fit but I'm anticipating it won't & hoping to get a response to my questions. Thank you to anyone who can help. Is there a source for NOS spider kit for a '70 Duster rear like the one I have? Thanks again. joel