I worked on a GTO a while back with a similar problem. It had excessive crankcase pressure. He had Total Seal rings, and one of the things Total Seal tells you to do is restrict the PCV valve to .060 (or something like that) or you'll suck oil. Problem is, with the PCV restricted, the crankcase can't breathe.

Anyway, he restricted the PCV, as per instructions, using an old Quadrajet .065 jet in the hose. After a short drive and a blast through the gears, the rear main seal was leaking. We cleaned everything up, I removed the PCV restriction, and the problem went away....

But the other thing I to his car is what I did to mine.

Years ago, my setup was stock, with an oil cap on the driver's side and a vent tube to the Ram Air pan on the other. After a blast down the 1/4 mile, the dipstick would always be about 1/2" out of the tube. Not enough ventilation to relieve the pressure at 6000 rpm.

I now run double breathers (on stock valve covers), both breathers have 5/8" nipples. I ran two 5/8" heater hoses to the back of the air cleaner, and into a T (easily found at any hardware store for a couple bucks), then the T goes into the back of the air cleaner housing. Now under full throttle, any crankcase pressure vented using TWO breathers. At part or full throttle, the carb is pulling crankcase fumes through the T in the back of the housing.

Basically the same system as the factory did, but instead of a vent tube on the passenger side going to the air cleaner, there are two vents.