You make some interesting points, however the more that is found about the effects of fallout, the harder it's going to be to sell that.
At the end of the day, fallout is the killer with this one. It spreads far beyond its initial blast radius(especially if the Soviets hit ICBM sites with nuclear bunker buster bombs) and makes areas it touches unusable, possibly for a rather long time. Hence, why you would need bunkers to send the populace too, and so on.
As for ABM... the things I've heard aren't encouraging, especially in the 60s. Simply put, unless we started getting ABM satellites(don't know if militarization of space ban has happened yet, but if so that presents an obstacle) we can't really shoot enough nukes down to save enough people. Even with them, enough may still get through, although maybe not, depending on their effectiveness.
Granted, the public doesn't know the latter, but the former? They'll learn that, as even if the government tries to cover up research like that... well, lets just say journalists can be very annoying at times for the state.
With all that in mind, interesting food for thought. BUT, would what you suggest be cheaper than its Flexible Response counterpart?