Does anybody know the part number to the rubber plug bellow the cap or somethin i can use to plug it up. coolant is leaking out of it.

 

You mean where the factory coolant return line from the heater core attaches in a stock 3rd gen F-body (or USED TO attach, in your case)?

If so, they sell rubber hose nipple plugs in the HELP! bubble packs at most auto parts stores. It'll probably be either 1/2" or 5/8".

Keep an eye on it every few months- they are NON-reinforced rubber and can deteriorate over time. Fortunately, they're cheap and easy to replace.