The R154 is a great box, I just think it has an oiling issue..... I would like to see it have an oil pump in it like an E series box, then you could fire some more lube at the thrust washer...
How does a cromoly thrust washer help? The stock cast one is not being "smashed" to death, it gets squished so hard by the thrust input that the lube oil is quickly squished out.... then it gets hot and bonds to the gear, then it gets riped to shreds as it shears off the keyway pin. You can make it "stronger" and "harder", but that does not address the problem.... splash oiling to a washer that is buried between a large diameter gear and a flat plate.
Metalurgically, cast iron is a great material for a thrust washer like this.... not too brittle, not too soft, and retains oil very well.
Wish I knew the solution.... my customer has lost faith and we are not adapting a built GM TH400 auto to the 7mgte.