try a few other cam grinders, possibly just contact them on the internet, and ask about cams with similar lift/duration and their recommended valve spring spec/settings. You might have trouble checking the tension, and absolutely don't 'wing it' - as you might get to the point of the coils hitting one another/binding, and that'll be potentially catastrophic (I've seen an engine push the valve stem so hard against them it ripped the groove/lip out of the collets and the valve dropped into the cylinder, ended up jamming, smashing through the bore wall, twisting a rod, you name it, all at semi moderate rpms too!)

If you are stuck for a tool to measure the valve springs, if you have someone with a drill press, you can mount everyday bathroom scales under them, and use a lump of steel in the jaws of the drill that helps keep the spring centred (not too long or it will hit the scales of course!) and then you twist the lever handle on the drill press down until the spring is XXmm high (whatever the installed height on the head is) and then look at the poundage/kg on the scale. It might not be the most dead on accurate thing in the world, but it _should_ get you close enough to see whether it's out big time, or at least in the ball park.

I haven't played with a lot of datsuns but I have done a fair few high revving minis, I mention this because they are similar sized valves. I can maybe look at what I used to run (I'd have to dig out some old paperwork, or worse case, dig the heads/springs out of the shed, as I held onto the 'good' stuff, it might offer some sort of a ballpark as to the seat and nose loads??

.4mm sounds right as far as valve clearances go. Apart from preventing thermal expansion from lifting the valve off the seat, it allows a harder/longer contact and helps the exhaust in particular transfer heat from the valve to the seat and off and away, which is good to prevent detonation (to say nothing of ex valve lifespan) - and it'll improve it above and beyond whatever tiny amt of lift you lose.. From memory the rockers are a 1.5:1 ratio, so 0.4 (15-16 thou in the old money) is pretty right - used to use around 13-14 on the minis (which were 1.2-1.3:1 rocker ratio std, but sooner or later went up to 1.5:1 when budget allowed for something better). If you allow for the rocker ratio difference, conceptually it's actually the same 'actual' clearance in the system.

If you pick up a couple of hundred rpm (not a thousand, just a couple of hundred) from moving the gaps from .3 to .4mm it's a _very_ strong sign that the valve springs are too soft or not preloaded/shimmed enough.

If you wanted to check if it's running out of fuel, don't do it for a long stretch, because it might be on the threshold of detonation as it leans out, but it's easier to 'find' it in top gear or the highest gear you can safely/practically get up to highish rpms in. In first and second, the motor just doesn't spend enough time revving before the upshift, whcih gives the fuel pump time to top up the bowls. A longer drawn out run in a higher gear will have the fuel starvation show up a lower and lower rpm, and the dropoff will or should likely be more pronounced.

But based on the footage, and the difficulties of a 'round the world' diagnosis, I don't think it's fuel.