What are the specific differences between the Remington 700 actions used for the traditional belted magnums (such as the 375 H&H) and the Rem 700 actions used for the 300 Remington Ultra Magnum ?

Hammer

The magazine box,magazine follower and the feed rails differ.

Guessing the magazine box and magazine follower are easy to change.

How about the feed rails ?

Hammer

The magazine box and follower are very easy to change but the feed rails must be done properly in order to have proper feeding.

Along with the feed rails needing some attention, the bolt face needs opening up since the beletd mags based off of the 375H&H are of the .532 rim diameter, and the RUM has a .534 rim diameter.

I put together my 7MM, 300 and 375 RUM with just openning up the feed ramp wider to accept the 404 Jeffry case. No changes were made to the feed rails at all. I cut windows in the ADL mag box to allow the shoulder to spread wider in the mag box thus allowing three in the magazine. I used the original follower without any mods at all. It feeds slow or in a hurry with no problems at all. I am happy with my RUMS. Good Luck. Olcrip