I have a guy that wants me to fit a 375 Ruger barrel to his current 300 win mag; bot face, extractor, feed rails, etc???

Follower

Fitting it to a Ruger? Or Win M70 action? Pre-64 or post?

Bolt face is the same as normal magnums, .532"... Magazine length is the same. Follower might be an issue, but it depends on which follower is currently in use... Test and see... Do NOT alter the feed rails, unless it's for a pre-64 - a different design there, as I am sure you're very familiar with..