The Rem 721 IS long enough for your project, Its also the strongest action Remington ever made.
If you want a 300 go for it. Talk with your local gunsmith, on your rifle all it takes is running a chamber reamer in the chamber and opening up the bolt face.
Depending on the type reamer your 'smith has, he may not have to take the barrel out of the action.
If would be a lot cheaper then rebarreling assuming your barrel is good.