The ones in the front corner should definitely be blocked off - remember, the water pump only pumps X GPM. Water takes the easiest path - if there is a shorter, less restrictive path(in this case out a front hole and back to the return right next to it), it won't have nearly the cooling going to other parts of the head.
It looks like they were trying for a "cross-flow" type design where the coolant comes up the edge of each cylinder and straight across the head between the two valves, then back along the top side. The holes are probably sized such that the water pump pulls relatively equally from each, so you don't have a cold front cylinder and hot rear one.
Despite that, the rearmost cylinders are expected to be hotter than the rest(lack of cooling? Or just burned oil?) if you look at the piston-to-cylinder clearance specs.
