And, I truly don't think the WCT could ever be referenced as a venue equal to a Grand Slam. Those tournaments were usually 8 player events I think.
McEnroeisanartist is right, the WCT Dallas event was bigger in significance in the 70s than either Australia or Roland Garros. It offered considerably more prize money & attracted considerably better players, even though it was only an 8 player field(best of 5 throughout though)
Remember, Laver-Rosewall in the '72 WCT Final is probably the most significant match in television history(it made tennis a viable option for television)
When Newcombe won the '74 WCT Finals he called it the biggest win of his career(& he had won many majors at the time)
That's what's tricky about this GOAT stuff, standards of greatness are different in other eras.
Borg & Evert, arguably the 2 greatest claycourters ever, both skipped the French once during their prime years in order to play WTT.
Clearly the French did not has as much significance as it does today(which isn't to say Borg's achievement isn't amazing, just on a purely tennis level, but even he would say that his Wimbledons were a far bigger deal at the time than his FOs. He never fell to the ground after winning a French. Also he was in tears after losing to Newcombe in the '74 WCT Finals, a rare show of emotion. I think we can at least agree that the WCT Dallas event was bigger then than any 'masters series' of today.
And that Dallas WCT event(on carpet) was held only a few weeks before the French in those days, which shows how hard it was to peak on different surfaces those day. Very poorly organized sport in those days, players' needs weren't valued.