I run the G body, and haven't had problems breaking the trans. I don't have down bars in the front but i do have a front motor plate, and i run the softer stock (not poly) rear trans mount. The only time i broke a trans case was when i broke a drive shaft and it pushes forward, actually i was running clips on the rear u-joint and it pulled the threads on the 5/16 bolts which cause the drive shaft to jump out of the rear yoke and go forward. Broke the trans in 8 pieces, the only thing holding it in the car was the converter, i put it back together with another case, didn't hurt anything and i was back in business.
Zip.