GM makes a Special Torque Converter Restraint Bar that attaches to one of the Bell Housing Flange Holes and it acts to keep the Torque Converter from sliding back and forth and thereby causing damage to the Hydraulic Pump Hollow Shaft Tang and/or the Soft Cut Seals in between. Instead....For now...Use a Bent Mild Steel Coat Hanger stretched aross the face of it and threaded through two opposing Bell Housing Bolt Holes.
Please remember this going forward...
This is a Transmission... It is an Object that has Internals that can Behave and Break as easily as a Dropped Swiss Pocket Watch. So... Take your time...
Slow is SMOOTH...and SMOOTH is FAST.
If you can find one... use a Larger Size Flat Steel Washer to spread around the Holding Force under the 15mm Bolt Head going back into the Broken Case Tang... you should still be able to use some of its Holding Pressure...
BUT ONLY IF IT IS THE LAST FASTENER OF THE (11) BOLTS TO BE TORQUED DOWN.
So Please... Be Gentle with it. You should NEVER have to use Violence to get ANYTHING else done during the rest of this work.
EDIT:
Almost forgot...
In case you encounter anything that wants to "fight" back in the near future even in the slightest, if you post your Questions or Statements about what the problem is before attempting anything that might be hasty, many here will jump at the chance to help you out.
For example... This difficulty you had with extracting the Transmission Cooler lines... A while back, both
@Mounce and
@Mooseman posted their suggested remedies for this problem over in the Tool Talk Forum that may yet prove a useful and very handy tool to have:
https://www.amazon.com/dp/B00AZW8PDC/?tag=gmtnation-20