Yep, pull the cylinder head; again. (you should be able to reuse everything, because you obviously couldn't have run the engine very long; just double check all of your water grommets and oil passage O-rings before you "reset" it back down)

I would only do the front cover, IF it was leaking and needed to be resealed anyways. (I assume the engine is in a truck chassis = means pulling radiator, fan and hub, oil pan, air compressor, P.S. pump, etc PLUS How will you set backlash on idler gear, with all that rocker arm valve spring tension, SO you will need to loosen everything on the top end (of the cyl. head), anyways = might as well just pull the cylinder head, because all the valve train and jakes will be off or loosened already)

Edit: I should add, You don't have to totally remove the cylinder head(from the truck) either = it just needs to be lifted over the alignment dowels and set rearward about 1"-1.5" = just enough to install and tighten the plug, with a 90 degree allen wrench. You got use some clean, short 2x4s, wrapped in clean towels to set cylinder head, while still on engine block; then it would be stable while you installed and tightened the missing plug. {you might be able to install the plug, thru the cam hole in front cover, too; while being lifted}

 I would also add to use a flashlight and a mirror to double check nothing falls on top of pistons, also = do this 10 seconds before you reset the head down.

(C15 cylinder head.jpg)

(CAT3406ECylinderHead.jpg)

C15 cylinder head.jpg (18KB - 101 downloads)

CAT3406ECylinderHead.jpg (81KB - 101 downloads)