Originally posted by MI2600
The oil breather cap on the oil filler tube should be for air intake. The metal cap on the back of the block, next to the distributor, should be connected to the carb via a pcv valve, and draws air out. As stated, if this is missing or plugged it will cause pressure build-up as you described.

Chevy used two systems: Open and Closed. You have the Open type.
THANKS TO EVERYONE THAT REPLIED! With the information you provided I found how to test it in my shop manual. I will let you know for sure if this fixes the issue after this weekend...