Brother I have a 383 also and I broke my trans bellhousing twice also. I think my reason it happened, still dont know for sure because I did a lot to fix it, was my flexplate. I had a 400 flexplate, meaning it wasnt neutral balance. It had that weight on the flexplate. MY 383 rotating assembly was a balanced kit. IT is a common mistake because all factory 400 cranks are externally balanced. It is a little harder to find a balanced/neutral flexplate for a 400. Cutting the weight off made a big difference in the smoothness of the motor.
Just to make sure though I also installed an ultra bell bellhousing. You dont realize how flimsy those 350 bellhousings are until u cut one off. That thing is weak.
I also installed a new balanced beefy driveshaft. Just in case that old original driveshaft was part of the culprit.
I also installed new body mounts. You would be suprised how flimsy a g body frame is by itself. If your body bushings are shot, your frame is actually moving more than it should.
After I did those things I havent had it happen again.
SO I would check your assembly and figure out if it is internaly balanced or externaly. IF it is internally balanced check your flexplate for a weight. If there is one on it, it needs to come off.
But I would highly recomend getting an ultra bell. IF you dont find out the reason though, an ultrabell isnt gonna hold it all together. You need to find were your vibration is or where your twisting is. My engine didnt have a major vibration when I had the wrong flexplate on. But once I took that weight off it was really smooth. Thats my bet of whats wrong.
Hope any or all of this is helpfull.
