That was an O.S Giken effort and nothing to do with Nissan. Nissan produced the cross flow LY heads though for the L-series. Which later would be the template for the RB30 SOHC engine.
The 4 cylinder L-series got the LZ configuration LZ20B which was a twin cam motor and very exotic. That was a factory effort.
Why was the Prince 6 never going to cut the mustard? The S20 motor that was derived from the Prince GR8 engine was very capable and responsible for the C10 GTRs winning many races. Not to mention Prince R380 was able to defeat Porsche's on more than 1 occasion.
I think people underestimate how special these engines or cars were for the time period. A PS30-SB configured with the right cams, diff, light weight features would have been a very quick car for it's time. Since each car (Z432-R) was different it's hard to pin down it's performance metrics, but I'd imagine they could have been quicker than Phase 3 Falcon's.
https://en.wikipedia.org/wiki/Ford_XY_Falcon_GT#GTHO_Phase_III
Which everyone seems to covet no end... but really just a big V8 in an average chassis. Great in a straight line, not so good in the twisties.
I dunno, I think the S20 sounds amazeballs at high RPM and it doesn't look slow either!