I definitely wouldn't put a 7 1/4 in there if you can find a 8 3/4. Keep in mind you can also use the earlier B body 8 3/4's if you move the spring perches and run a little more backspacing on your rims. The early B rears (no later than '70) actually work better if you run 17's or 18's, because the backspacing will match better with what you can run in the front (5-6" backspace).
Keep your eyes out for an 8 1/4 too, it would be a big improvement over the 7 1/4, and not always as hard to find or as expensive as the 8 3/4.
Just remember that if you go with an 8 3/4 or 8 1/4 you'll have to shorten the driveshaft a little and you'll need new u bolts and shock plates.
