I have a '97 K1500.... L31 (350 5.7L), 4L60E, F44 chassis (14 sf rear)..... 234,610 miles as of this writing, still on the original driveline. I bought it in 2013 with 176,003 miles and it has been my daily on and off... Currently my daily since 11/2021 with no end in sight. It does have issues here and there, but it is comfortable to drive, and paid for.
Some mechanical things I have done over the course of ownership...
SFI to MPFI conversion, while apart for intake gasket replacement
Ignition module upgrade
Replace original distributor
Replace all brake lines with nicopp
Replace fuel lines with Nylon
Replace the entire HVAC system (compressor, evaporator, consenser, lines, heater core, blend actuators etc)
Replace radiator and water pump
Replace manifolds with shorty headers
Replace steering box with red-head unit
Replace variable effort power steering pump with '96 PS pump and pressure hose
2 full brake jobs (lots of towing and mountain driving)
2 (cheap) fuel tanks
2 fuel pumps
1 set of 2017 z71 wheels, 18"
Driveshaft re-balance after the OEM sound deadening layer grenaded off while driving
1 set of bed bolts from Totally Stainless
Probably some other odds and ends I'm forgetting about...
On the upside, I have addressed most of the problem areas of these trucks, and feel confident about driving it just about anywhere. They are great trucks, but with 200k+ on the clock, you'll have to be on top of maintenance and take care of known problem areas before they become an issue.