I posted this over in the Engine forum and would like some more input.

I noticed a puddle of water under the 383 today.
Got to looking and traced it to just above the pan rail and at the bottom of #6 cylinder. It's on the outside and about 1" behind the block drain. Right at the bottom of the cyl where it turns to straight vertical.
Looks to be right at the bottom of the water jacket.
The water weeps out along about 2" following the bottom of the jacket when pressurized. I scraped the paint off and can't see a crack. Which doesn't mean a whole lot....
The block was acid cleaned before the rebuild.
I'm hoping it's porosity and not a crack.

Is this fixable?

I've go an AL radiator and water pump too.

What do you guys prefer for a stop leak type fix?

I used this ceramic stuff..it works great...i think moroso makes it

My Olds heads needed a can of K-W Permanent Metallic block sealer. Worked good--but note that you must drain and flush the cooling system to REMOVE THE ANTIFREEZE before you install the sealer. Once the sealer is in 'n' hardened, you put the antifreeze back in.

This isn't where I bought mine (I went to the local CarQuest) but it shows a photo of the can.

http://www.amazon.com/K-Permanent-Metallic-Block-Seal/dp/B00030BF7O

The actual can of goo I bought had been on the shelf too long, all the "active ingredient" was solidified in the bottom of the can. I spent 1/2 hour or more with a pair of pliers, and then a hammer, mashing up the solidified crap--broke it into small lumps, then to about the consistency of aquarium gravel, then beach sand, then finally got it to about the consistency of flour or cornstarch. Dumped it back into the watery stuff it's packaged with, and dumped the whole works into the radiator. Took 2+ hours of engine running to seal the leaks--but they did seal, and have had no problems since.