Yeah, there was no IWGP heavyweight title until the 80s.What they had before it I cannot seem to find, tournaments I guess.
From its inception in 1972 until 1975, I am unaware if they had any champion. However, in 1975 NJPW became a member of the NWA, so they only used the NWA titles. There was a brief time in the late 70's when WWWF actually rejoined the NWA and Antonio Inoki held the WWWF Championship, although not recognized today.
A lot of early NJPW was dependent upon NWA wrestlers coming to Japan, which is why they were comfortable using the NWA Championship as their world title. Of course they decided to start their own in 83 and the rest is history.
But for information's sake, the main title in Japan when I was born was Pacific Wrestling Federation World Heavyweight Championship and the champion was Giant Baba.
Looks like Jack Brisco was the NWA champ and Verne Gagne AWA champ. Not a bad field of champs for my birthday.
EDIT:
I didn't want to start a new post and hijack this thread, but Stopspot's post about NJPW championships really got me thinking. I knew that they were an NWA affiliate and contrary to what I said earlier, Inoki actually did not win the WWWF title due to the NWA affiliation. WWWF and NJPW had a working agreement at the time.
Doing research, the tournaments which began in 1974, may have been the major "championship" of the promotion, aside from the NWA. The tournament winner even was awarded a belt, which is why Hulk Hogan claims to this day to be the first IWGP Champion when he was in fact was simply the winner of one of these tournaments. Furthermore, it looks like in addition to the NWA titles, until the IWGP belts were established, there were a constantly changing array of interpromotional titles that NJPW used.
An interesting read:
www.wrestling-titles.com/japan/newjapan/