As mentioned, the Soviets had to stage their long range assets forward from their normal bases for even one way missions in most cases. The forward bases were relatively austere and they could not sustain the forces forward for any significant length of time. Depending on how long the buildup to DROPSHOT was, the number of aircraft deployed forward could be anywhere to a small percentage of the force to a large percentage. Naturally the more aircraft forward the better chance some of them have of making it to deliver their payloads to the USA one way or not. Don't forget that even if forward staged most if not all of the bombers do not have their weapons on board and ready to go, nor are they fully fueled. Bombing up and fueling will take some time, and these forward airfields will be among the first targets to be hit by the USA.
While of course one would not want to count on it, it would not be ASB for the USA between not all Soviet bombers being staged forward, airfields being hit before all of the deployed bombers taking off, and then the US/Canadian air defenses that actually no bombers reach their designated targets in North America. You could see some bombs dropped "at random", possibly on any urban area of any size that a bomber sees if it can't make it to the designated target or simply dropped somewhere rather than see it not used at all. Of course this is the maximum best case situation, but not ASB in this timeframe.
In terms of air defenses, SAMs in the 1955-1960 timeframe were just coming in to deployment except around Moscow. The SA-2 first deployed in 1958, so if you are looking at the 1955-1960 (DROPSHOT) timeframe there may or may not be any at any given target site. PVO doctrine was very heavily on ground control intercepts (GCI) and like everything else, heavily based on top down authority. The initial clearance drops would have a good chance of not only destroying or severely degrading airfields, radars, and GCI centers as well as trashing radio communications in general. Again, depending on the tensions at the time of attack, a relatively small percentage of the fighter force will be either in the air or on ground alert, and aircraft availability could be as low as 50-60%. Any fighters that get airborne and survive will be looking for someplace to land and that field will need mechanics, spare parts, and ammunition not just a place to refuel. Degradation of the air defense will be severe, command and control in bad shape.
The initial strikes could suffer pretty bad losses, but with exception of Moscow and perhaps a couple of other major targets, the subsequent waves will do much better. Don't forget that the USSR will have to deal with strikes from the USA, and to a lesser extent the UK, that are coming from almost every angle. In 1955-1960 the USA has bases in North Africa that can strike from the south, as well as carriers in the Med and the potential of tactical aircraft carrying out nuclear strikes from Turkey. You have strikes coming from the UK, and again possible tactical strikes from everywhere in NATO. Of course there are the strikes from the USA east and west coast, the Navy Regulus subs and carriers.