7.25 swap

I'm guessing that what you are calling a prop, is what we call a driveshaft? There are 2 different sizes of U joints, 7260 and 7290. Unless the 489 case was in a high performance car, it very well may have a 7260 U joint yoke on it. If that is the case, simply buy a 7260 U joint and put it together. If by chance the 489 has a 7290 U joint sized yoke, you can buy a conversion U joint that is 7260 on one side and 7290 on the other. Way cheaper to do that. Clean the treads for the u joint straps really good before attempting to install the bolts. A new U joint should come with new straps and bolts.