These heads were made before 1973. They are not made for modern fuels. To get any reliability out of these heads, you will need to have hardened exhaust valve seats installed. Not under the springs.
What is the installed spring pressure?
7 degree locks/retainers would be better on this setup.
Off topic.. I have a set of 487X heads in for repair. These were on a pickup truck. Mild 383, I built three years ago. Customer supplied heads.
The exh seat above the heat crossover has receeded about 1/4". The guide is shot. On the other head the center exh seats are about 1/8" below minimum height. I was impressed that they were not cracked. So much for soft seats..