Is it a center-feed action or staggered feed action?
I built a centerfeed 6.5 Swede. Swapped out the barrel from a 30-06 and the cartridges fit just fine in the magazine.

Changing a staggerfeed to a DBM is difficult. For a centerfeed rifle, all you need is the bottom for the magazine, as well as the magazine surround frame and to make sure the stock is inletted for the surround.