Can a 7mm RUM be converted to .300 Weatherby without too much trouble? I know I'll need a new barrel, but was wondering about the bolt face being too big or too small?

The head on a RUM is .534 and the Wby is .531. Should not be an issue. Issue I would question is if the feed rails that were open enough for the RUM would control the Wby. That I have no clue

i think the feed rails will be to wide, you may have to go 300rum

Yes, that would certainly be the better move, with near identical ballistics and no belt.

The magazine box for the "standard" magums is different than the RUM box. If you have access to a standard magnum magazine box, you could switch and see if they will feed properly or if the feed rails are too wide for the Weatherby. Some actions when converting to the RUM fron a standard only require the RUM mag box and feeding is okay, the next one needs the box and some rail work, so I suspect you could get lucky, otherwise as mentioned the easiest would be to rebarrel 300 RUM, then all you need is the barrel.

If you are dealing with a Rem 700, you may find that it feeds 300 Weatherby better than the RUM rounds and even better than from the 300 weatherby mag box such as found the weatherby sendero. You have to just try it and see.