Quote "This only leaves 3 thrust bearings that could be causing the problem. One behind the reverse ring gear, one behind the rear planetary and one in front of the rear planetary."
Did this trans originally have a thrust bearing between the reverse ring gear and the low sprag hub? The sprag hub is the part that bolts into the rear of the case.
There are 3 different sprag hubs... all differing in height.
Tallest hub is for the C6 that uses a thrust washer (not a bearing) between the rear ring gear and the hub.
Next shortest hub is for the E4OD that uses an open faced thrust bearing between the rear ring gear and the hub.
Next shortest yet hub is for the E4OD that uses a closed thrust bearing between the rear ring gear and the hub.
The E4OD parts can be retro fitted to the C6 but you MUST use the correct height hub.
If you used a bearing between the rear ring gear and the sprag hub.... and used the tall C6 hub... you WILL have a stack up height/end play problem.