Valve Cover Question

[Jason Bourne]

1972 C10 V8

Currently there is an unfilled hole in one of the engine valve covers. See red arrow in the pic.

My question is what should I do with this hole? Does having this open affect performance at all? There is a hose on the other valve cover that connects to the carburetor. I assume this is some type of vacuum advance. I'm thinking the open hole would be causing a vacuum leak but the truck runs ok for the most part.

I am having an issue when the truck warms up, the engine revs drop when sitting at a stop light in drive to the point of the engine almost dying. When the truck is warm the revs increase in Park or Neutral and the engine idles fine.

Maybe there was some type of PCV valve that was there before? I was told the current engine is a 283 that came out of a 1964 Chevy II.