If you get down below .12" wall thickness, then you are getting toward "whippy;" stay around .15 (which would mean a muzzle diameter of about .64" (2x.15+.338) would be ample. So that is very similar to aalf's .65".
I would think about a #2 or #3; I have a #3 Douglas on a Mauser in 35 Whelen and it is a good match.
