Rokushin Kaikan is definitely based on the real-life karate organization, Kyokushin Kaikan. It's a kanji difference of 六真会館 vs 極真会館, with Rokushin meaning "Six Truths" and Kyokushin meaning "Extreme Truth". (Kaikan just means "meeting hall" basically)
Also, there's an organization in Grappler Baki that's another reference - Shinshinkai. It leaves off the "kan", but given its presentation it's clearly the stand-in for Kyokushin. Shinshinkai is written as 神心会, which means "Divine Heart Association". It could also be considered to be a plan on the other real-life Karate style, Seishinkai (聖心会 - Sacred Heart Association)