Is the action a Pre 64 Winchester mod. 70, or a newer manufactured action in .300 HH? If it's a pre 64 in .300 HH, keep the rifle as factory as possible - it could be worth quite a bit to pass on. Original Pre 64 Mod 70s in 300 HH were not produced in big quantities at all, making them all the more desirable. Unless you have significant attachment to the rifle, you might be better off selling it, and using a newer action to build a .300 PRC.
From the other perspective and realistically speaking, you shouldn't have an issue with the cartridge overall length, providing the magazine has some room for the slightly longer cartridge of the .300 PRC. The case head diameter is the same between both cartridges, so there's no need to worry about the cartridge seat compatibility between them. If you're looking to have a nice custom rifle built on the Mod 70 action, you should be able to make it happen.
Should you move forward with the project, how much of the current rifle do you want to maintain while turning it into a .300 PRC? If you wish to keep the same stock, balance, feel etc, then you'll want a new barrel in a very similar profile to the original. That might not the be the easiest to replace. Likewise, if the current gun has iron sights, do you want to maintain those, or would you be ok with losing them and just mounting a scope?
