I have an 84 300 and I can't find an aftermarket breather cap to replace the stock cap.. I'm running an "Offy" 4 bbl intake and HD exhaust with the PCV plumbed directly into the intake,,All the caps I've tried are to big of an I.D. for my stock valve cover...Any ideas..The next move is to take the stock plastic cap and fab a hose fitting to the air filter can. Not a very clean look..

 

I've run aftermarket breathers before and didn't have this issue. Try specifying the grommet ID when you search.