actually i am going off of personal experience on this one, the first engine i ever rebuilt was a 68 pontiac 400 and i ended up with the same problem. luckily i had become pretty good friends with the guy who did the machine work on my motor(it's amazing how fast you make friends when i costs you $1000) he is actually the one who gave me this hint and then i was reading the next months car craft magazine and they did an article on firt time engine rebuilds and sure enough there it was for pontiac motors, if you leave this oil galley plug out there will be no oil to the right side head, i have never read a manual on how to rebuild a pontiac motor so i don't know which ones are informative or not but i do know from personal experience that the oil galley plug is behind the cam plug, if you remove the cam plug from the rear of the block it is a thread in plug behind that. good luck that is were i would start especially if you don't recall installing one, you should howevere be able to remove the distributor and use a telescoping mirror and flashlight to look into the hole and see if there is one in there, or feel down there with your finger i remember being able to feel it with my middle finger when it was missing, it is about 3 or 4 inches into the distributor hole towards the rear of the block, good luck