Come look at the TWO transmissions I have waiting to get shipped to FLT as cores in exchange for a Level 4. Oh yeah... I have an N/A cammed/stalled/tuned (very mildly) 5.3L powertrain. Stock transmissions will not, I repeat, WILL NOT hold up to punishment forever. When it's time, it'll break. Have friends that have gone through 2 - 5 rebuilds with cammed 5.3s and 6.0s.
I have access to a cammed 6.0, FLT drivetrain RCSB (Z71). After loads and LOADS of 4wd burnouts, the dang Level 5 just won't break... though it's broken a few G80s & ripped apart stock gears.

I was a believer in stock components til my second one bit the dust. Gonna fix this the right way now... FLT it is!

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