Or maybe the case had a fracture alraedy(maybe hidden under the paint) and the burnout finnaly made her split.

I have been through the same thing with my F-Body car and finally called Ed Quay. After talking with him we were able to solve the problem.

Had (2) problems, The pinion shaft of the rear end was not in line with the crankshaft and I also had to install limiters on the engine block to the subframe to stop the front to back motion of the engine. It didn't make sense to me at first but Ed told me the transmission was breaking when I let off the gas. Well that has been about 10 years ago and I haven't broke one since. Maybe you will be as lucky.

Make sure you dont have any bent or mis aligned splines on the driveshaft. I broke a 350 TH two times and it looked alot like your trans.

Looking at my Energy Suspension poly motor mounts , I noticed that the hole the bolt goes through are oval , not round. I imagine this would allow the engine to twist and rock. Possibly a contributing factor.

I have afriend that has a turbo 350 that looks just like that one, as we speak. Ive seen this 3 or 4 times and each time there was no midplate and poly was used on at least the trans mount. I believe they are breaking due to front to back movement. In the burnout your torke is on and off especially if you feather the throttle. Don't minimize it because it happened in the burnout. They say the burnout is the hardest on almost all racing components on your car.
Make sure you have a well mounted midplate and solid motor and trans mounts. As it moves back and forth it seems to find a weak spot and snaps the belhousing like a pencil. I believe your frame is ok, its your engine and trans moving within your frame, its doesn't take much.
Take care and at least you have some cool pics to show your kids.