i'd just run the standard rifle spring and buffer in your a1/a2 stock setup and go from there. it should be just fine. if after testing you find you need to modify then make changes.
those who choose to add weight to an adjustable stock system are doing so because the carbine buffer thats usually standard in them is several ounces lighter than whats in a standard A1/A2 rifle buffer system - 3oz vs 5.2oz.
both systems share the same wire diameter of the spring, the main difference is the coil count - 44 vs 39 - to make up for the length difference in the tube.
the bushmaster 450 uppers are already free floated, so that modification is already done for you.
as chris said... if you want to make a positive change from a stock AR platform, look first to the trigger. The trigger will be your next big bang for the buck thing.
good luck with your build