Take a look here:
I don't know how tennis used to work those days, but they played some huge clay tourneys twice that year (or this is a bug on ATP's website?)
Look at the players Nastase won against in all those finals (Panatta, Borg, Pilic, Orantes, Emerson etc.).
I think it's the best season ever of a player on clay.
He played 12 clay tourneys and he won 9.
(1 final, 1 sf and 1 qf).
He won RG, Monte Carlo, Madrid, Barcelona (twice !), Rome, Cincinnati, Florence, Gstaad.
Also, the overall season is outstanding !