It takes a lot to crack a TH-350 case. The fact that you did indicates that the case was subject to torsional stress. usually the cause is using solid engine mounts and or a solid transmission mount. Both are a no no. Another cause is a broken engine mount or broken/cracked frame.
Broken bellhousings seem to tend to come up on Small block-TH-400 combinations more than big block setups with either the TH-350 or TH-400 trans or SBC-TH350 combos. I don't know why. At any rate you need to eliminate the cause of the stress on the bell housing/case. May need to go to frame connectors and add a torque strap to the left side of the motor to limit torsional movement. Do not use solid motor mounts.