threaded plug on rear of block

Do you happen to have Napa"s part number because they can"t fid it at my local store nor could I find it at Sears. I put a tap wrench around 5/16 square stock and that would work if they weren"t so frozen, afraid of breaking the handles. Thanks for the offer, if I can"t get them out after heating and more cursing I just might take you up on the offer! The entire engine came apart like butter untill...My first engine build at 46yrs young so I'll be asking many more newbie questions I'm sure.