Another thing is and this is a personal opinion here.
I would rebuild the 60e, and get the vehicle going. But, I would then start looking for a usable or decent core for rebuilding 80e. And start saving for the 80e swap parts to do a 80e swap.
Solely for the fact I have destroyed SSSOOOOOO many 60es over the years with low to medium power engines( 3 or 4 with just a 4.3 v6), that I would never trust one behind a 6.0 engine. I know they were behind them from the factory... but is still don't trust them.
I am coming to the conclusion that, when considering the 4L60E, it's the luck of the draw in terms of how long they last...The primary failure factors are:
1. Casting quality of the valve body
2, Casting quality of the forward drum
3. Metal used in the sun shell
The valve bodies exhibit wear rates that are all over the map in terms of mileage...I've vac tested hundreds of 4L60Es ranging from 40k miles at failure to well over 200k and the common denominator seems to be excessive wear in the TCC regulator, AFL valves as well as both abuse valve bore plugs, end plus and in some cases the 3-2 control valve (96+).
I also see significant variance in drum sealing integrity...Here I'm referring to the input shaft-forward drum junction where the cast steel shaft is pressed into the case aluminum drum...The input shaft has three different hydraulic apply circuits passing through this junction and, depending on the quality of the cast aluminum drum, its female splines exhibit either no-minumal wear (passes leak test) or high rates of wear (fails leak test)...The drums with poor casting/accelerated wear invariably present with dramatic 3-4 clutch pack failure. This on top of a relatively weak direct clutch pack compared to peer units from Ford and Dodge (4R70W / A518 respectively).
Lastly, the piss-poor, soft metal the factory chose for the sun gear reaction shell (aka 'drive shell') leads to numerous 4L60E failures. Upon teardown, these shells are either decapitated, or very nearly so, presenting with extensive wear at the neck or neck splines stripped (no reverse, 2nd or 4th is the result).
Assuming those three things are corrected on overhaul (properly reman valve body, aftermarket performance forward drum and/or shaft and Sonnax Smart sun gear reaction shell, assuming the transmission is otherwise built per the application, engine power levels and calibrated mechanically and/or electronically as needed, they can survive in high performance/RPM applications....
I am less inclined to use them in heavy duty applications where lots of towing/hauling/working with loads at least 15% more than what GM rated the vehicle to handle...At this point, I agree - a 4L80E swap is the way to go (perhaps paired with a rear-axle swap and suspension work to address any potential weak links elsewhere on the vehicle.
