Are any of you using Picatinny or Weaver type bases on your action ? I have the magnum short action and would like to find a source for a 40 MOA 1-piece base.

Nightforce offers both one- and two-piece bases in slopes from 10 to 60 MOA. The 20 and 40 MOA bases are standard items; several others are special order. www.nightforceoptics.com

As I understand it anything that will work on a Winny Mod 70 will fit on A Montana, same contours so they claim.
Straight shootin

Tha's partially correct. The front spacing on the long actions is .862 and the rear is available as either .330 or .862, which I assume covers both pre-64 and post-64 variants of M70 actions. Spacing on the short actions is .862 front and rear.

www.montanarifleman.com/actions.htm

You might check www.kenfarrell.com, there is also another company making picatinny bases as well but they have not finished the design yet - should see something on it before Shot Show.