300 RUM , 30-338 Lapua improved ,30-378Wby , though the Lapua and bif Weatherby case is realy pushing the limit of what anybody should have in a SS action , but it will fit.
I would rebarrel it and get a longer barrel , you will also be happier with the better barrel quality of a good after market barrel.
the bolt face wil have to be opened up , and the mag well fitted to the new fatter case.
Sell the rifle, and buy the one you want. The 30-06 is a great caliber, and you shouldn't have any trouble moving it. You can find Ultramags on the auction sites for about $500.00 & up.