You can use any rear axle you want as long as you can get rims with the right backspace to keep the wheels in the wheel wells.
The 5th Ave's and Diplomats with have an 8 1/4 rear axle, which is a good rear axle for even moderately hot small blocks. But, it's over 2" wider overall than the rear axle in your car now. Which means you'll need about 1" more backspacing on your wheels if you want them to stay in the same spot in the wheel wells. If you want to keep your current wheels, you'd need to make sure you have about 1.5" from your tire to the quarter panel right now. If you do you can go the the F-body 8.25" rear and still have enough tire clearance. If you don't have that much, you'll probably need rims with more backspacing.
I have a 68-70 B body 8 3/4 in my Duster, it's 3" wider than a stock A-body 8 3/4. But I also have modern style rims that have 7" of backspace. And Dusters have wider wheel wells than Darts to begin with, so I had more room to work with.