You could take it to a carwash and pressure wash it.
BradMph is right. If you overtighten those cover bolts you could distort the casting. The Haynes manual specs the torque for the valve cover bolts at 60 inch pounds. That's only 5 foot pounds. If you have a torque wrench for putting the head back on you can check if it will go down to 5 ft. lbs.