I had a leak once, little bit of "the right stuff" and the leak was gone. I didn't pull the heads and I knew the next time, even with a o-rings on the sides, it still needed a good coating of silicone.

It's very nice each spring when I put the pushrods back in and the rockers back on I can make sure they are in the cups of the lifters without messing with lot's of silicone to seal it back up.