What specific modifications are necessary on the M70 action to get a conversion to 300 UM to feed properly? Thanks all!

Kinda' depends on what caliber rifle you're starting with. Maybe just a mag box, maybe a lot more.

We need a little more info before we can fully answer that question.

As 'toomany tools' stated, mag box, action rails, feed ramp, extractor and bolt face will most likely need to be altered, but it all depends on what M70 you're starting with.
If it's a 375, all of the above will need to be altered, if it's a 416, you may get away with only a few mods.

Cheers.

Aren't the new ones available in 404 Jefferys?

That would just be a barrel away.

375 H&H to 500MDM. Stock M70's in 300 UM are hard to find lately and the ones that rarely pop up for sale are pricey. Just wondering what needs to be done to a stock 375 H&H.

Rebarrel a .375 H&H to .300 H&H and your feeding problems are over. No further modification needed.

It does not go as fast though.

Does it kill anything deader than dead?

Which would you rather have, a rifle which shoots a cartridge with a fantastic velocity but with a tendancy to jam, or one which fires a cartridge slightly less fast, but absolutely flawless in function?

With 90 head of African plains game to its credit, my Model 70 .300 H&H has never failed to feed perfectly, nor to bring down what I hit properly with it.