What special tool are you talking about? A straw that fits in your tube where you check the oil?
You don't need a breather cap on an EFI engine, but you do on a carbureted engine. You can use a closed oil cap or do the straw thing if yours is an EFI engine. I don't think an 80 Mustang engine is an EFI engine though, so you will need a breather cap on the driver's side in addition to your PCV hole on the passenger side, otherwise pressure will build up and you will have engine damage. Bad idea.