All M70 actions from 270 to 375 are the same basic action, so conversion to 416 Dakota can be done but it won't be straight forward.

You would need to get the magazine box and follower that Winchester used when they chambered for the 300 Remington Ultra. If you want reliable ejection, mainly of a loaded round, then the rear receiver ring needs to be altered as Winchester to for 375 H&H.

Bolt face needs to be opened and extractor altered. The rails of the action also need to be changed for the larger diameter case. The ejector bolt stop needs to be shortened to allow the bolt to come back further.

In short, you need a gunsmith as distinct from someone who basically chambers and fit barrels.

Unless you have your heart set on a 416 Dakota then 416 Remington would be an easier option. More gunsmiths will have had experience changing M70s to 300 Wby, 7mm STW etc and these are the same basic case as the 416 Reminton.

If are not set on the 416 Dakota but want the bigger case capacity it offers over the 416 Remington then necking the 375 Rem Ultra to 416 might be better since the brass situation will be better.

The 375 Rem Ultra case is a few thou bigger in diameter than the 404 Jefferey, which is the basis for all the Dakota rounds up to 416 Dakota, but the Rem Ultra has a very slightly rebated rim which is the same diameter as the various magnums such as 7mm Rem, 300 Win, 375 H&H etc. Some people would consider the rebated rim on the Rem Ulta brass to be a negative, however, the amount of rebate is very small.

Mike