[Tom Goldman]

X2 on mounting.
Case cracking in this area is usually due to flex in mounting , lack of a mid plate ,and engine limiters. .....Just gets worse when you apply more power.
I just had a customer with a stock mounted LS engine with a glide experience the same problem . got away with it until he boosted the power.
If you already have a mid plate and limiters, do not use a solid trans mount , as there can still be enough flex to crack a stock case. Driveshaft alignment should also be checked .
Bottom line , stock cases are at least 40 years old , many may have been in wrecks and subject to unknown stress before being used for racing. ...They have served valiantly ,but are very weak compared to the aftermarket cases .