340 head gasket questions

The really important coolant holes are the ones in the rear. The coolant flow should be front-to-back through the block, up into the heads and then rear-to-front through the heads. The other holes are likely there just to get the casting sand out. The ones that are plugged may just have casting trash in them and are not really important for the main cooling flow. Too much open area up front is actually a problem.