Would it be ok to put an oil cap breather on a 1977 Trans Am (400) in place of the stock oil cap?

Valve cover breathers have nothing to do with the EGR valve. As long as you have a PCV valve in one valve cover, it is OK to put a breather in the other side. On my engines, I put a pcv valve on one valve cover, and 2 breathers on the other. Without breathers of any kind, you can blow out seals in the engine, causing oil leaks.