Are you primarily interested in just having a rifle with a shorter barrel, or are you trying to create something that looks like a factory carbine?
Either rifle (a 24" pre-64 sporter or a 24" or 26" classic rifle) will work just fine cut down to 20" (pre-64 carbines were 20"), but you will lose some accuracy and muzzle velocity. Personally, I would go with a pre-64 or post 2008 featherweight at 22" and save the cost and trouble of cutting and recrowning a longer barrel. The factory featherweight will be lighter and more accurate than either generation of model 70 cut down to 19" or 20".