Is the Winchester M70 Pre-64 action to small to handle the 300 RUM?

not if you find an H&H action, but that might be costly. You can however find a classic in that cartridge.

If you do such a conversion, magazine box width may be an issue. The RUMs (and other fat cartridges) don't always feed and/or eject reliably from standard magazines.

I was more worried about the thread shank diameter. A Winchester has a barrel thread shank of 1"-16tpi a Remington is 1.062"-16tpi.

Some people think that a .378-based Weatherby case, which has a body diameter of .582 compared to .550 on the RUM, is unsafe in a Remington.

The RUM in a Winchester action has a .015" thinner chamber wall thickess than a .378-based Weatherby case in a Remington action.

My question has anybody put a RUM case in a Winchester action?

The action I have was originally from a 300H&H rifle. There is no difference in size between the std action and the magnum H&H action. The only difference is the rear bridge is notched out on top, the rails are opened up and lengthen to accept the H&H cartridge, and the boltface is opened up.

I will probably have to build a mag box since the cartridge has less taper.

Check this out;
http://www.brownells.com/aspx/ns/store/ProductDetail.as...DED%20MAGAZINE%20BOX