I don't have any direct experience with what you ask, however I've played with other downloaded calibers.
I ran the following report from Quickload for a 26" barrel, Nosler 200gr AB, 2700fps and a load density of at least 90%. I tried first with the 180gr bullet, but it wasn't heavy enough to get good load density and burn %'s. If I where playing with this, I'd try the Retumbo. All of these loads are low pressure, so keep an eye on the necks of your brass for soot marks (black powder residue due to lack of initial expansion). You can usually minimize this by having a good amount of neck tension and very little if any jump to the lands with your bullets.
Again, I have no direct experience with the 300Rum and downloading, so take this info for what it is worth.