yes, this variant of pr.667 was never built, time of classic SSBN-s, like George Washington, came. this variant was very unusual and carried too small quantity of missiles, instead pr.667A, which carried 16 silos.
about pr.958, it was perspective type of 3rd generation submarines, which was created to serve as a "AWACS" for smaller killer-subs, descendants of Alfa-s, pr. 657 and 693. pr.958 would have extremely power sonars and lowest level of noise. this idea was similar to seawolf tactics of penetrating to soviet "bastions" but was created earlier, in 70-s. this wolf-pack of submarines 958+657/693 should operate near US costs, in ohio-s (and others ssbn-s) patrol areas and against multi-layer NATO ASW. Akula ssn firstly was a variant of this project.
but building of this expensive subs was not on forces to soviet industry in 1970-th. later, this project was re-created as 4th generations subs, with same tactic, but without killer-subs, individually. but falling of USSR destroyed all this plans. graney project was created instead of 4th generation pr958 , as cheaper and more-purposes submarine with same
sonars and tactic.
also, pr881 "Mercury"(planet
) SSGN, pr.935 SSBN, projects 957 and 833 SSNs was created, but was never built within 4th generation of soviet subs.
also, you can see more information here: deepstorm.ru and paralay.com. sorry,only in russian. contact me with icq if you want