Looks like you have already answered your own question for the most part.

When it comes to converting to cartridges that were never offered by the factory you can pretty much guarantee you're going to have to do some fiddling with the magazine to get it to feed reliably. You've already done the hard part of determining if your current magazine will hold and feed the new cartridge, so it's just going to be a matter of tweaking the feed lips as needed. That said, the magazine used for the belted magnums and the .30-06 family of cartridges were different, so if you can find one you might make your life a little easier if you can find the .30-06 box and follow.