There is a large misconception about having to have a aftermarket Case, I have seen alot of blown Trannys and I have never seen a broken Case. Now most of the time when a Case does Fail it is when the guy Uses a Motor plate with a completely stock case and the Bellhousing breaks and the crack went up and into the case.
I have broke 2 before with ultrabells on them. the crack went around the area where reverse comes in with presure so the fluid would come out mainly when i set the trans brake and in reverse. the bell was not broke in fact i used the same bell on both, when i went to a stronger case it fixed it and now i use the same tranny with my 557 that i ran with my 355 winsor. anyway the guy wants an aftermarket case so that is what i am looking for, but i will tell him that it's his ultrabell and plate that may be causing the problems.
ARE A PRODUCT OF THE MOUNT OR THE MOTOR PLATE THAT HAS MOVEMENT WHEN TORQUE IS APPLIED EVEN IF YOU THINK IT IS SOLID WHEN BUILT.MOST CHASSIS SHOP RUN A BRACE FROM THE REAR OF THE TRANSMISSION CROSS MEMBER TO THE CHASSIS TO PREVENT THE PLATES FROM FLEXING AT LAUNCH.I HAVE HAD A LOW 8 SECOUND CAR FOR YEARS AND NEVER BROKE A STOCK CASE.BUT YOU ALSO MUST REMEMBER THE POWERGLIDE IS A VERY OLD UNIT AND YOU NEVER KNOW WERE THE CASE CAME FROM