There was a good reason the 240Z engine had the resonanance problem in it's early iterations.
Prince was acquired by Nissan some time before the Z car was commited to production. Prince had been in dire straights and ceased car production and decided to focus on just delivery trucks. They had no money for a suitable truck engine so they licenced one from Mercedes in Germany. Their Japanese bank facilitated the acquistion by Nissan/datsun.
So when Datsun needed an engine in a hurry (see below) they ended up using the Prince/Mercedes light truck 6 cylinder which had never been intended to be reved above 4-5K rpm. I've always suspected that the lack of feedback from Japan to the US and the delay in producing a resonance free crankshaft was that the Prince/Datsun engineers had to go back to Mercedes for help to get the problem solved in a hurry. Loss of face and all that plus confidentiality contracts kept this relationship under covers to the public at large and presumably BRE and Sharp as well.
Remember, at the corporate level, the Z car was a response to the very positive reception the Toyota 2000 GT had received. Datsun had initially negotiated with Yamaha to produce a 'modular' engine that could be built in 4 and 6 cylinder versions. The Datsun contract with Yamaha had Yamaha keeping the design ownership and Datsun paying a premium price to Yamaha to build the engines.
When the Toyota 2000 GT was cancelled by Toyota (as it was too expensive to build and sell well), Datsun cancelled (shelved?) the Z car project as no longer necessary and Yamaha was left with some great DOHC engine designs but no sales.
So Yamaha approached Toyota with the same engine design that Datsun no longer wanted and presto, The Celica GT was born and Datsun had a problem. They had a car but no engine. That is why they resorted to the Prince SOHC 6 cylinder light truck engine when they frantically restarted the Z car program now required to compete with the Celica GT.
I loved my Z cars. They were smooth and bullet proof, but then they were designed to be used 'hauling the freight' for years and years so why wouldn't they. Every Z I owned was totally reliable for well over 100,000 miles each. Then I bought a Maxima. Quick but dull. The V6 just didn't zing.
Japanese car company politics, the direct influence of Japanese banks and the Japanese determination not to 'lose face' created intense yet very private competition between Toyota and Nissan/Datsun. To find out what was happening between these two companies one needs good sources. In Germany as well as Japan.
BTW. If you happen to park a Mercedes 230/250/280SL alongside a Datsun 240/260/280Z, both with their hoods open, compare the layout closely, and especially the later Bosch mechanical fuel injection models. As Schultz would say, "Veeeerrrryyy interestink".