Farmall B antifreeze in oilpan

I have a 1945 Farmall B that my students and I are in the process of restoring. When we drained the oil the first thing to come out was nice bright green antifreeze, followed by the oil. I have pressurized the radiator and looked for leaks around the sleeves (suspected lower o-ring may have been bad on sleeve) and have not seen any leaks around them nor are there any coming down the oil galleys from the head (suspected headgasket).

What I have seen in a very faint hairline crack in the block on the inside of the crankcase just behind the carburetor. I saw a couple drops of antifreeze there when I first had the oil pan off. I wiped everything dry before we began searching for leaks. After sitting for the weekend I noticed a few drops of antifreeze under the tractor right below where I saw the drops in the crankcase. Sure enough they we back in exactly the same spot I first saw them! It looks like there is some weeping coming through what looks like a crack on what would be the bottom of the water jacket into the crankcase.

Any ideas of how I can stop this leak short of replacing the block? Here's the picture I was able to take of the leak. What look like little bumps in the casting are actually the drops of antifreeze/water. The apparent crack starts from the far left drop and goes horizontally across to the right. Thanks.

You can either find a used block that is not cracked, or have it professionally repaired. There is a place not too far from you that does that kind of work.

http://www.midwestcylinderhead.com/

How did you pressureize the radiator when the radiator is vented thru a tube on the left side draining down below the fan. The crack on the outside wont hurt and can be easy repaired. I would remove the pan then with coolant full look to see if there is a leak at the bottom of the sleeves as the rubbers sealing them can leak. This is a wet sleeved engine. There are many places for soolant to leak into the oil but the rubber washers at the bottom of the sleeve is the bigest cuse. Remove the pan and let it set for a couple of days to check the sleeve for leaks. The radiator is not made for putting pressure as that engine uses the thermo-syphon system and is not pressureized.