Thats not what Bob Sharp says in his Datsun Competition Preparation Manual that was published in the 1960's. He says in section 4-14 in regards to the Camshaft:
"Although the stock Solex camshaft has proven to be quite efficient at 6800RPM, more horsepower and higher revs can be realized from a re-ground unit. The Solex cam has enough material to grind almost any profile that will give horsepower readings of 185 at 7200 RPM's."
and in section 4-16 in regards to the Valve Springs:
"The stock valve springs are totally adequate for use with the Solex camshaft if the revs are held to 7000 RPM's. If a racing cam is used and the revs will be exceeding 7000 RPM, it will be necessary to install a spring strong enough to accelerate the mass of the valve train at such a rate that the rocker arm will stay on the camlobe at least 500RPM above the desired red line."
I'm not saying that manual is gospel, but Bob Sharp was pretty successful and if that manual was written in the 60's and he was running it at high RPM's back then. Well, we must have better components & bearings nowadays that would allow you to run at least at the same higher RPM's.
If you don't have a copy of that manual or haven't read it, you can download it for free at www.Datsun.org