I changed the spigot bearing in the crank and the throwout bearing last year chasing this noise to no avail. I spoke to an old hand about what it might be and he said it's probably just a bit of play on the input shaft bearing and it's no big deal.
It says in the ad it's an L20 and a GTX model, the 4 cylinder hako is G16 powered so I assume (without seeing engine shots) this is a long nose.
I like that it doesn't have flares, that's got to be a bit of a novelty these days!
Forget what the literature says, it is only a STARTING POINT
Many of your posts have the same theme that "X said Y therefore I should do Z" which just isn't right. Use the tools at your disposal to tune the engine as well as it will run and be happy. Your mixtures are probably never going to be 100% perfect because no two cars are exactly the same and because of the modifications your car apparently has.
Sounds like a great car but frankly if you are prepared to fix your current car yourself you would probably have it ready for RWC for under $1500, even including new frame rails.
I would highly recommend the Pierburg pump available at EFI hardware. It's quieter than a Carter pump, cheaper and out of the box pumps 2.7psi.
https://www.efihardware.com/products/261/lift-pump-340hp-Rollercell
There are a group of Prince Skyline owners in Melbourne which we call the "Prince Mafia" because of how tightly they have a grip on almost anything classic Nissan and Datsun among the local clubs. They prefer to stay out of the limelight for the most part and I would guess there are over 10 Skyline GTA and GTBs in that group.
I went and saw these in person and there is absolutely no way any of these cars justified their final selling prices. They were in good condition but nowhere near what I would call immaculate. On top of that I heard somewhere the series 1 was advertised for a while privately for $27k so I bet the owner is feeling pretty good about himself.