Well, there are more people to start with.
Excellent point re civil defence - anti-nuclear shelters and bunkers will be even more effective if it's a non-radioactive "nuclear" winter (actually caused by the impacts). That might, in fact, make a major difference. I hadn't considered that. America might well survive as a coherent, albeit drastically weakened, entity.
Yeah - by the 70s I think most of the NATO/Warsaw Pact countries (who are the ones taking the impacts to begin with) had enough infastructure in place to at least
survive a nuclear war, in the sense that NAm and Europe wouldn't turn into lawless cannibalistic wastelands overnight. The actual impacts are in the many-megatonne ranges, I think, and the tsunamis are going to be nasty but over quick; I can definitely see most industrialized nations making it through "intact", even if the Winter's going to be a bitch.
Hmm: when did the comet hit in the original? fall? Let's say the comet is detected just as it starts breaking up under the Earth's gravity in late August, one month before impact. Confirmation wouldn't take long - I mean, any organization with a telescope could see it - so what happens then? The Cold Warriors can't blame each other, horror of horrors, so I guess we'd see a sudden upsurge of "we must all hang together"ism. I don't think they could predict the points of impact until it was way too late - IIRC the current thought is that we could predict a straight-in hit about an hour before actual impact, way too late to do anything for the poor sods underneath, and the disintegration of the comet will just make that far worse. So they can probably have a date, but not a place. Mass hysteria, exodi, survivalism, etc; this may end up proving a bigger strain on some countries than the actual immediate aftermath. I'm also willing to bet that while the US and the USSR won't do anything overt - they're about to have
much bigger problems to deal with, after all - there will be at least one country that uses this to settle old scores: in 1978 I'd put my money on the PRC/Taiwan, Syria/Israel, and probably an african country or two.