Depends if you want to mess with opening up the bolt face to Mag dia. or get a new mag bolt.
If not , as others have said here, you can never go wrong with 338/06, 35 Whelen, or 9.3X62.
But if the barrel is in good shape don't overlook the 30/06 Ackley Improved. It is a simple reamer job with no other changes. The increased case capacity will allow use of the 200gr. and 220gr and other heavy for caliber bullets along with their great SD to be pushed to higher velocities with excellent results.
Like all the Ackley improved line, factory ammo can be fired in the improved chamber in a pinch with little loss of velocity or accuracy.
P.O. Ackley like Rocky Gibbs, was not only a great gunsmith, and wildcat developer, he was a experienced hunter with a lot of time in Africa. He knew what he was doing.
Good luck with the build.