can the holes be plugged if not hooking anything up to the port in the back of the head?
The nubs are solid. I believe the only ones they are drilled on, are the California cars... and with those, the nubs will have a hole in them for the air to come out of. Even if they are drilled (CA application), a person should be able to plug the end of the head where the air is injected.
When porting these heads, I grind the valve guide boss out as well. From the port exit, it's a straight line to the back of the bowl. I do that, and have bronze guides installed. The intake ports need a LOT of work as well... as they are only 126cc stock. I ported the hell out of my 289 heads back in the day, and they were a whopping 155cc.
Performance wise, there is no reason to use these old heads. The cost to rebuild them is almost as much as buying a name brand aftermarket set. The only way I would fool with a set of stock heads anymore, was if I was BROKE, desperately needed an engine running, they were in perfect shape, and needed zero rebuilding. Then, any grinding you do is pretty much "free" horsepower. Other than that, buy a set of Edelbrocks or something.