Had my car int he shop for what I thought was a leaky front seal. After pulling the trans apart and replacing some glazed 3rd gear clutches (another post). The leak was still there, they are now thinking the torque converter is cracked. Has this ever happened to anyone? This is in a 67 coronet, 550hp 383, supercharged efi. 4000#lb car. The converter is a 9.5" PTC unit.

Yep. B&M super holeshot cracked at the hub weld.check the converter with air pressure when you pull it out.

Just had mine fixed last week for a front seal leak and it turned out to be a cracked torque converter. Its just a 904 behind a 318 2 bbl. not sure of the manufacture of the converter. Had it rebuilt about 10 years ago with about 10,000mi. on it.

Seen it way too many times on 727s and 904s. But the 904s seem more prone to cracking the neck. Dont want to bash so i wont say the brand. But even seen a new one leak right out of the box.
Now we can pressure test them before installing.

how do you pressure test them? Why is it a common failulre?

I think mine failed becuase I had the flex plate bolts loosened and three of the four ears of the flex plate broke. I was at the track and heard a tapping noise at Idle that went away as i raised the rpm's. I tighted the bolts (didn't notice the cracks at first) and drove it home. the noise came back in a couple of days so I took a better look and realized the flex plate had cracked. In the mean time it developed what I thouhgt was a front seal leak but was actually a cracked hub.
This was in a 727 behind a warmed over 340 with 4.30 gears pushing about 3800lbs.