A blown head gasket can be minor enough that you won't get a cumulus cloud behind you. However, I wouldn't use any kind of a stop leak because the leak will only get worse. I would plan on changing your head gasket.
It's really not hard on the straight six. A warning though, make sure you check not only your head for warpage, but also your block. My 235 had both the head AND the block warped, so when I had the head milled, the leak was worse than ever...with a perfect head gasket. I didn't see that coming and was disappointed that I had to yank the 235 out of there.
