I did my stack at 129K when I did the EGR delete and turbo swap. It needed cleaning too.
The coolant should be changed every 100K anyway.
If the water pump makes it I'm at 199K now... My plan for 229K is drop the coolant, break down the stack for cleaning, replace the water pump, idlers, belt, fan clutch then put it back together with fresh coolant.
2007.5 GMC 2500HD, Gear Alloy 712C Injector wheels on 285x65x18 BF Goodrich T/A KO2, Cognito leveling kit, Cognito suspension and steering upgrades, Bilstein shocks, Amsoil bypass filtration, FASS 95GPH, Transgo JR, PCV Re-route, AFE cold air intake, Diamond Eye 3" down pipe, AFE DPF and CAT delete pipes, EGR delete by ProFab, Danville 3794 Stage 2, Katzkin leather, Mean Green alternator