Dear Gypsy,
I have the same concern as you as I too have a '59 Sno-kitten! I talked to Tucker Corp and they recommended 10W-30. I also spoke with Cooks Equipment back East (a Tucker dealer) and they recommended the same with a mention that a detergent-free oil could be best, so as to keep the 'scorings in the cylinder walls from being cleaned out and lowering engine compression possibly". (My local auto parts places do not carry detergent-free oil, so no luck there.) Both Tucker dealers I spoke with don't have much experience with extreme cold like we do in the Rockies so I'm currently using 5w-30 so it'll start easily. I've added some Lucas heavy-duty oil stabilizer to help with any compression loss and to keep the oil from running off surfaces while sitting. I used only 1/2 the suggested ration from Lucas so it'd stay viscous enough to start. Well I don't know if that was a good move or not from 10w-30 but I'm currently trying it. A magnetic block heater (stick it to oil pan) for starting may be your best bet if you have some real AC available for cold starts.
I also run SeaFoam in my oil to help clean out the exhaust valves of excess carbon. Additionally I had a case of bad fuel that caused my intake valves to stick (a very common problem with classic cars that sit unused). A good treatment for that is to dump SeaFoam down the carb to really clean things out (carb, intake manifold, intake valves). I add it to my gas as well, so as to clean the gas tank, fuel lines, fuel pump and carb. Supposed to be a wonder additive. If you've never done that before, it might be a good treatment to try.
All of this is my short-term plan until I figure out anything better to do! Oh yeah, I'm running Amsoil synthetic Severe Duty in the rear end 75w-140.
Send me a pick of your Kitten, love to see it!