Please help / antifreeze leak

I hope some one can give me some solid direction and advise with this problem I now face with my engine. I just had my 273 with steel heads rebuilt by a professional engine builder and we fired it up and found that we have a antifreeze leak in the back of the engine and it is dripping off the corner of the drivers side head. I'm having a very difficult time seeing where this leak is coming from.
Here are the facts so far. From what I can see is it don't seem like the freeze out plug on the head is leaking, and there is no antifreeze in the oil and there is no white smoke coming from the exhaust. The end stud for the exhaust manifold is in there solid. The leak is right on the corner of the head where it meets the block. It's very difficult to see the leak, but it seems it is getting worse.
I was told to retork the heads but I don't think this is the case. Some one told me to use the Barr's stop leak, and I really don't want to go down this path being it's a complete rebuild and I have heard alot of mixed reviews. The thing is the person who told me to use this stuff has been building mopar engines for forty years and well respected but I'm really nervous about this.