That's a good discussion above on Har-Tru courts (rublico) vs. red clay and also on Borg/Connors on clay. Connors preferred the "faster" clay and also playing in the United States. Furthermore, he did get Borg when Borg was 19-20, as opposed to 20+. Having said that, Connors was a great rublico player, so he should get credit for being one of the best rublico players at the time, along with Borg and Vilas.
Orantes took the US Open in 1975, and Connors beat a 20 year old Borg in the 1976 final. In 1975, Connors beat Borg in the SF. In 1977, Borg retired in the 4th round vs. Dick Stockton with a shoulder injury. (I don't think he ever lost a match to Stockton). In 1978, they switched to hard courts and Borg lost to Connors in the final, but Borg did have a bad blister on his thumb (thumb unjury) and he could hardly grip his racquet after pain shots.
I'll add that even M. Orantes was pretty tough on rublico (green clay). Borg beat Orantes in the FO final in 1974 on red clay, but Orantes beat Connors on rublico at the US Open final in 1975. Players such as Orantes, Vilas, and Clerc were some very dangerous clay courters back then that could give serve and volleyers fits on clay. Wood racquets on clay vs. modern racquets on clay have changed the dynamics some, plus they have supposedly made clay at the Fo faster than it was during the 70's. So, with wood racquets, slower red clay, and no Luxilon strings, blasting a great clay courter in the 70's off the court was a very tall order.
No one could manufacture a man that won 6 French Opens and 5 Wimbledon titles in a row. - Andrew Longmore