Thanks for all the info. Sounds like my best option is to rebuild the one in it myself, with the upgraded drum. Don't worry, I'll have an experienced transmission tech helping me. Or maybe I'll just yank it out pay him to do it.
This is assuming, of course, that this truck is still available when I get back from work next week. Man, trucks are hard to find right now.
How are you going to drive the truck? If you don't abuse it, the engine is stock and the transmission is property built, the 4L65E that's in it should be ok. I don't know about "good for life" since clutches in all automatics wear in time. I recommend the transgo shift kit installed on the conservative side, especially with respect to number of spacers under the 1-2 accumulator. I suggest none or one at max. I also suggest the "The Beast" sunshell and the Corvette servo. The soft spot on these is the 3-4 clutch pack. That is likely where it is slipping. You can "stack the pack" by using thinner steels and putting in an extra clutch. I'm not sure how I feel about this mod. On the one hand, more friction material is good but on the other, The thinner steels concern me. I'd go with redline clutches and Kolene steels. They aren't that much more expensive.
My truck has a small cam in it (219/228 .525/.525 on a 112 LSA). It has the 5.3. It had headers, a high stall speed torque converter and 4.10 gears. It's 2wd. It's a regular cab/short bed. I'm guessing the 5.3 puts out around 400hp/400tq. That's a guess. The 3-4 clutch pack in the 4L65E started giving up at about 110k miles. I had the unit rebuilt along the lines of what I mention above. I drove it hard. I managed to start pulling the input shaft out of the input drum, which caused an internal leak, slipping, and burning up the clutches (I limped it to the office). It hung in maybe 20k miles. I had it built again, this time with a billet input shaft/drum. I shattered the output shaft on a wide open throttle 1-2 upshift. I was done. I swapped in a 4L85E. Best thing I ever did to the truck. You can read about it here if you are interested....
A little less than two weeks ago the transmission went out on my daily driver. It's a 2008 GMC Sierra 1500, regular cab, short bed, two-wheel drive with a 5.3 and a 4L65E transmission. The 4L65E has been built twice in the last 50k miles. It has some nice goodies inside it but I keep finding...
gbodyforum.com
If the engine in the truck you are buying is totally stock and if there will be no abuse, you should be able to rebuild and beef up the 4L65E and have it hang in there. I won't try again. If you tow, avoid towing more than it is rated for. Either way, watch your transmission temperatures. You want it running 160-180. Frequent drain/refills will also help. I drain/refill the 4L85E in my truck every 10k miles. It's cheap preventative maintenance. It's overkill, but the fluid stays bright cherry red.
I don't know about swapping to a 6L90 but my guess is it is pretty straight forward since they came in those trucks. The biggest thing to overcome would be having the segment swap flashed into the TCM/PCM for the different transmission. That can be done by mail or email if you have or buy the programmer and pay the tuning service.
We run a fleet of 4x4 3/4 ton trucks from 2014-2020 that have the 6L90E in them. They seem to be pretty stout. Most all of those trucks have 180k-215k miles of commercial service on them. The transmissions in those trucks get serviced at 100k miles and again at 200k miles. So far, I've only had one fail, at 212k miles. It was in one of our trucks in Dallas. It broke down near a Chevrolet dealership so they took it there. We ended up just having a rebuilt unit from GM installed so I don't have any forensic data on what went wrong with the original unit.
Sorry for the long-winded response. I hope it helps.