1. A 223 should clean up a 222 chamber.
Issues - If the chamber on the 222 was to the large side, a 223 may not clean it up totally, especially if it is truly a match chamber. as said above, there may be a slight ring where the shoulder was on the old 222, but should not hurt any thing. Chances are good that it would not be an issue.
If done improperly, there could be head spacing issues or the chamber off center to the bore. That is always a chance that is taken when enlarging a bore or even boring a fresh chamber.
2. The 222 was designed with a 1-14" twist. They had 52 grain bullets in mind for it. Of course they were flat based bullets. A 1-14" twist barrel should be able to stabilize a 55 grain bullet so long as it is not of a long design, such as a boat tail or an exceptionally long hollow point. It should stabilize any of the flat based spitzer style bullets (soft point). Of course you will need to shoot your rifle with them to try it out. Some will and some will not. You are reaching the ragged edge and slight crown issues and velocity really come in to play. If you are going to shoot 40 grain bullets, there should be no issues with any of the 40 grain styles, unless it is of a non lead core. Some of those bullets get real long for their weight. Even so, there is a good chance it will stabilize them.
3. The Winchester model 70 is a good rifle and should take any thing you can come up with, on this side of even a proof load for a 223 - which is way over safe pressures.
Good Luck and Good Shooting