I'd be looking for a 1992-2006 SS Classic model 70 to work with.
Perzactly what I'd do given the options presented. The reason to buy a Pre-64 70 is for nostalgia/historic significance/collector value, and in that sense, I'd want to keep her intact. Seems a pretty expensive route to go just for the action. Otherwise, I personally think the 70 Classic is a superior action, mainly due to its availability in SS. I prefer the safety and trigger on the M70 vs the Ruger MK2, though there's nothing really wrong with the M77.
