I would think you should be able to change out the follower and adjust the feed lips of the DBM, IF none are available. Check with Midway USA or Brownells or Numerich for what you need. I believe the springs for the followers are the same whether it's a magnum or std cartridge.

I have a rifle like you describe.

since you mentioned the 7-08, and you have a long action, here's what I suggest. I would go the 280 Rem or 280 AI (A standard SAAMI round now) since it's a better match up for the long action. Brass is easy to come by and not expensive, plus the 280 AI is within about 100fps of the 7mm Mag with less recoil and less powder usage. Recoil will be way less than the 300.

For the record, I would try to see how the cartridges feed through the magazine before ordering anything else. There's not a snots worth of difference between the mag follower and the '06 based one. I have both so i know this for a fact. I had changed my '06 to a 338 WM and the rounds actually fed BETTER from the '06 follower. I did have to adjust the feed lips a bit, but that is just trial and error.