Having 'done' a few Bears (?TU95?) in the 70's, I cannot recall the cruise levels but think they were indeed around 280/300, sufficient to give the 'Frightening' some challenging formation flying
I suspect that since they did not 'recognise' Western airspace a flight plan/ NATS route would have been the last thing on their minds, and 'cruise levels' when not 'playing' with their friendly interceptors would probably have been optimum.
Amazing trips, though, up to 4 hrs 'on-station' in the Atlantic with the Russian fleet and then all the way back home, or as YS says, on to Cuba or Conakry. They were pretty heavy when westbound through the Iceland/Faroes gap having just tanked.