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.