C6 failure diagnosis

'68 C6 behind a truck 390 failed when it "slammed" into reverse with choke on. Initially loud "clattering" noise from trans, then nothing.
Now, I get drive in 2 and 3 only, no 1 and R, but I can feel the low-reverse clutch engaging when selected. It's due for a rebuild anyway. "Slamming" into reverse had been a problem for awhile, cured it once by "unsticking" the boost valve spool, but it came back. Control pressure was OK.
Looking at the schemo, I figured the only thing that could cause drive in 2 and 3 and no lo-rev would be a broken low-reverse one way clutch inner race. I've never heard of this before, but it would've had to break all five attaching bolts to the case; either that, or the inner race hub itself disintegrated. It was below freezing when this happened, and steel gets more brittle as temperature decreases.
Anyone ever see this before?? Every trans shop guy I've spoken to never has. If it's just a matter of five broken bolts, I'm betting large that I can replace the inner race, put a master kit in it and be good to go. Opinions?
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