63 Dodge Dart, now let me see;)

I've heard the 8" will work if it's the correct width. Be careful some have pinion offset causing 1 axle to be a few inches shorter than the other also measure drum to drum if it's close. Mopar 8 1/4 or 8 3/4 are great just short supply and too much $, unless you find a deal.
You can start the project with the 7 1/4 for now but if you are hard on it, it will self-implode.