I would do it. I have a rebored model 70 and a Springfield, in 35 and 375 Whelen respectively. JES recommends a minimum of .200 over groove diameter for a rebore. A 9.3x62 would require at least .566 for a muzzle diameter. Your 721 should be that and then some.

It's not too difficult to fit a 700 stock to a 721 if you want to go that way.

Chronographs, bore scopes and pattern boards have broke a lot of hearts.