If you can see bolts on the  the rear end cover , as you look toward the front of the car it's a 7.5. Who told you that a 1979 Ford Mustang could be used for parts ?  Are you sure it has the C-4 and not the C-3. Except for the engine , transmission ( some times) it has no parts in common with the Pinto.The Mustang that you are thinking of is the 1974-1978 Mustang II....The 1979 is the start of the Fox series of cars....Of which had very few parts in common with the Pinto..

the owner had  a spare c4, radiator, and a few other parts i mostly bought it for that and the rear end. he was gonna put a 302 in it. on the car it does have a c3. so saying its a 7.5 rear, would that be okay with the 302 in my pinto or should i keep looking for an 8inch?

(my plan was to take the 2.3 in the pinto now n swap it into the mustang when i put in the 302. could make a lil bit of money off of it as a running car.)