If your 30-06 barrel is in good shape, it will be a easy and cheap job to rechamber to 300 Win. mag. Just rechamber the barrel, will not even have to remove the barrel, open the bolt face. Sometimes, not always, you may have to open the guide rails for the larger dia. 300 mag case.
As for overall lenght, they are both the same so no problem with the lengh of the magazine. I have done about 5 on my shop lathe.
RELH