needing a new rear end and advice

If I were to do it again, I would have gone with a B Body 8 3/4 instead of an A Body as it closer matches the offset for the front when it comes to wheels. However, since you don't seem to want to go too crazy with your car, I'd suggest an 8 1/4.

So far my Dart with a 273/904/7.25 has held up fine. Granted, my 273 is tired and probably makes as much power as a rebuilt slant.