You can change the valve seals without removing the heads. You just need a good compressor. I have done that several times. Just a note, after reading several comments, no one mentioned the PVC should be connected to the carb base and not "FULL" vac from the intake manifold, that is usually for the brake booster. I'm surprise the motor dose not run ruff at low RPM's, when you run the vac line into the valve cover without the PVC. The PVC is design to cut down the amount of vac being drawn from the valve cover. If you are getting blowby with the PVC hooked up, then you have an issue, as some here touched on. Blowby, without the PVC hooked up, I believe is normal. Again, as mentioned by others, do your checking before tearing apart your motor.