How bad does it leak? I ran one for 20,000 miles with a crack like yours, but it was only 9 27/32 to 11 13/16 inches long (a little arrogant American humor for your pleasure smile ).
Don't bother trying to weld it. J B Weld will slow the leak down if not stop it completely.
I added some stop leak in the cooling system in mine (no J B Weld) and it slowed the leak down to a very manageable level. These cooling systems are not supposed to have a lot of pressure on them (0 or 4 pound cap), so its pretty easy to slow or stop a leak.
I am still kicking myself for scrapping that motor. When I took it apart, there was plenty of life in the bearings, rings, cylinder walls.
Carl

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission