75 Dart 360 Sport factory crank sunroof

When I bought the car the guy I bought it from said the car originally had a 8 3/4 rear end and he swapped it out with a 8 1/4 rear end. I have since bought a 8 3/4 rear end with a sure grip differential because I was to the understanding the larger rear end would handle more HP. If the car came with the 360 and the 8 1/4 rear ended, perhaps there's no reason to change it? Maybe the 8 1/4 rear ended will hold up just fine? Any opinions?