My preference is to not run plain cam bearings with over about 900lbs spring load, but I freshened some 572-13 heads last winter where the builder had the heads set up with almost 1200lbs open pressure...... running on plain bearings.
That’s over 2.5 times what you’re running........ plus....... he’s running 1.7 rockers, and the pushrod side of the valvetrain sees the spring load x the rocker ratio.

Your 450 x 1.5 = 675
His 1200 x 1.7 = 2040

I guess what I’m saying is....... I highly doubt it’s the spring load itself that’s the problem.