I would not rev the 6CTA in our trucks over 3K. Cummins says they see a lot of snapped crank shafts due to differential harmonics. No damper in the world can manage that. The marine motors mentioned use a different crank.
The 5.9L springs for a 6BT - Bosch P7100 work and they're down rated in our pumps. So a 4k rpm spring is about 2950 in ours. It's what I put into most of the trucks we've built. They're happiest under 2800 rpm - ish... every motor is different. Keep in mind, it's not just springs... it's the entire enchilada that keeps your motor safe and generates power AT HIGHER RPM - without air and fuel mods, I'd stay at stock mil spec 6CTA RPMS.
It's pump timing, fuel loads/injectors, turbo/air, air restriction and then of course, exhaust restriction and valve spring capability. Get a EGT and boost guage before you do fuel/air mods, please.
Most of the trucks we've done use a bigger turbo, different injectors, different pump calibration, different intake with a charge air cooler, stacks and a PAC brake.