In a typical V8 configuration, you should have a molded tube coming off the back of the carburetor spacer plate. The narrow end of the PCV valve is attached to the end of that. The fatter end of the PCV valve is pushed into a rubber grommet on your passenger-side valve cover.
well, i guess i have a problem. when my motor was rebuilt the mechanic never put that into place. i have a '69 351W.
the way mine is set up is he took and put a rubber hose about 1 1/2" long off the back of the carburetor and clamped it off. and what i have on the passenger side valve cover is just an oil filler cap.
Hook up the PCV valve. There's always a certain amount of blow by past the rings. The gas fumes from this blow by will be stuck in the crankcase and will mix with your oil, breaking it down if you don't get the fumes out of there ... the job of the PCV system.