I have not found that to be the case at all.
What I find is that people are worried about running high open loads and the cam going flat, and often aren’t running high enough loads for the rpm they’re turning.

You didn’t say if you were running 1.5 or 1.6 rockers, but 1.6’s would require more load.
I don’t know which heads you’re running, but if it’s something that uses big heavy offset intake rockers...... that needs to be taken into consideration as well.

In a BB bracket race build, looking for say 7200rpm capability, with a reasonably fast rate design lobe...... I’d be looking to be in the 420lb open range........ maybe more depending on how fast the lobe is.

Lobe design is a big player.
The first cam I ran in my 383 went 7k clean, no problem.
Second cam, which had a little steeper closing ramp struggled to get past 6500..... with all the other valvetrain components the same.

It’s a nice idea that you don’t need high spring loads on flat tappet cams for high rpm use........ but many times, that’s just not how it plays out.

Obviously I don’t know for certain if that’s your problem, but based on what you’ve outlined as what you’re running for spring loads...... I wouldn’t rule it out.