The bottom metal's are different between the stocks. Meaning, that you need to use a 721 metal with a 721 stock, and a 700 metal with a 700 stock. The actions will use either one, depending on which stock you use.
So, in other words, you can put a 700 stock and bottom metal on your 721 action, but the stock might require fitting for some parts (bolt handle angle factory cutout). Or, if you wanted to, you could put a 721 stock and bottom metal on a 700 action, but the stock might require fitting for the bolt handle angle cutout. But the actions will interchange and work with either stock/bottom metal. The bottom metal is just dependent on which stock you use.
From what I can remember, the stocks shouldn't require any inletting, maybe just some massaging of the bolt handle cutout notch. Other than that, the 2 actions and stocks should be identical (other than the bottom metal).