Just for conversation, both of the ST-10's I had in my '79 t/a, both split the cases under the torque of a healthy 455.
I had Paul Cangelosi build a custom fine spline M22 4 speed consisting of new Autogear supercase, steel midplate and new gearsets (optional ratios), along with other high quality hardware he uses. Trans is rated for 600ft.lbs. torque and price was $2400.
No more grenaded ST-10's.
https://www.5speeds.com/