Some of the holes in the 250 type heads have very little meat on the bottom and easily will break through unless a stock length screw is used. A regular 1/4 - 20 screw or bolt can be used if you just apply some silicone sealer to it before installing. It will seal fine. A stud may be a better idea because with a screw you need to drain down the engine of coolant if you want to remove the valve cover.
Either method is ok and not a major problem as long as you let the sealant cure a bit before refilling with anti freeze.
