Mine: 98 k2500 6.5 TD, original 4L80E.
460,000 on the trans.
Has outlasted 3 engines, which is not really saying much when talking GM 6.5 engines....
Tows a 9,000 lb travel trailer. Have made several east/west/east/west/east coast hauls.
Engine is "pumped up" and is 100 ft/lbs over the 4L80e max input torque rating of 440 ft/lbs.
Transmission and 14bolt full floating rear axle is about the only thing on the truck that has never given me trouble.
I'm running the OEM "aux" trans cooler.
I have the transmission tables tweaked for compression braking, TCC lockup, line pressure and shift points.
That said, you can swap in a 5 speed. Not hard, but not exactly easy either. There's a "how to" on the forum. Definitely "do-able" if you know your way around a tool box.....