Need advice

I've got a 7.25 Suregrip with a 2.92 ratio but want to change the ratio to a 3.23. I also have an open 7.25 with a 3.23 ratio that I can use for parts. Will the ring gear and pinion interchange between the two rears? Do I have to change both the ring gear and the pinion or just the ring gear? This is going in a 62 Valiant, 225, with an A-833 non-overdrive. Tires will be 15". Any suggestions will be appreciated.