Dude- slow down!
The 8-3/4 is a much better axle than the 8-1/4. Plus you will have to get a new driveshaft to match. Dart driveshaft won't work. Talk to Dr Diff about getting new axles for your existing 8-3/4; then, if you want to keep it relatively "low $" get the matching drum brake kit for the rear. In the end you will come out ahead. Then if you ever go higher power or want to swap gears the 8-3/4 will be there already.
