I did a search and read that people are claiming that rebarreling a standard length magnum action to 375 Ruger should be bascially a drop in fit.
Has anyone actually done this to a W70 Classic action and how easy was it? Any modifications required to the feed rails. etc?

we've done it on the AR rounds, and should work perfectly for the model 70

Can the action still hold three down easily?

depends.. if you have the weird spring, with the "hump" in it, probably not, but if you change it out with a normal one, probably