With the 35 whelen not being a very fast cartridge you should be fine with the 1-12 twist. Either 1-12 or 1-14 twist should privide you with acceptable accuracy. The 1-14 is a good medium twist rate that would provide you with the widest range of acceptable bullet weights to use. If yo plan on mostly reloading with heavy bullets then the 1-12 might be a better choice. If you are going to use mostly factory ammo then I'd definetly go with the 1-14.

I would advise you against turning down a blank yourself unless you are going to send it off to be stress relieved. Turning down a blank will introduse added and unwanted stress to the barrel and that's not a good thing. A #4 contour is plenty big (stiff) enough for the 35 whelen especially if you are going to chop it down to 20 - 22 inches.