How do you fix engine blowby coming from the valve covers on a Chevy 350?

The problem could be valve seals or excessive piston ring blowby. You could try changing the pcv valve on the valve cover and check the line going to the intake plate to make sure its not stopped up or the intake plate isn't stopped up.

However, it most likely means that the piston rings are worn out are broken. Time for a engine rebuild. That is the only way to fix that problem.