Converting a rem 700 .308 into a 300 win mag?

I was going to build a custome .308 rem 700 starting with the action. I wanted to start in .308 and eventually convert it into a 300 win mag when I shoot the barrel out. (Kinda like the m24). would it be ok if I started with a long action to make it into a .308? ( I see a lot of 7mm rem mags for sale cheap). Plus im lefthanded and every used 700 seems to be in 7mm. Anyway would this be a good path to go on?