Way way before cams were nitrided, or tool steel lifters, or special polishing of the journals.......
Look at what MP was recommending for springs for the .590, .620, .690 cams and what the open spring pressures were.

It was way more than 420 open.

I’m not saying those recommendations are right or wrong, but just think of all the motors that were built according to the MP manuals and ran those exact combinations of parts with no problems(or noises).