I just changed my push feed M 70 from 300 H&H to a 300 Winchester Mag, and now it does not feed all the time.

Do I need a different follower for the Win Mag or need to open up the rails a bit?

Wow, If I had a 300 Win Mag I would want to convert to 300 H&H!

I'm not a gunsmith, but I think that the shorter case is fatter in the front. If the rails were optimized for the thinner front end of the H&H, and since the shorter case begins to cam out of the rails later producing a sharper exit angle, the rails probably need opening at the front end somewhat.

I had to open the rails on two of my rifles to get better feeding after conversion.

Don't start by changing the rails or you may regret it. It is most likely an issue with the mag box. You might try adjusting the top edges of the box by gently bending them outward.