tune question
Someone else just had the same issue:
https://ls1tech.com/forums/lt1-lt4-modifications/945913-about-timing.html
If you just noticed it after you changed the rear and it was working fine before, then that's almost certainly the problem.
Transmissions usually slowly start slipping. It's a gradual process. So it was mechanical you would probably have noticed it getting worse over time, not starting all at one point.



