Recently bought a 98 454 dually... ...looking for some good mods and reliability adders.
The FIRST thing to do is to assure you have at least as much power, safety, and stability as GM intended.
Verify all the "tune-up" items--cranking compression, spark plugs, plug wires, cap 'n' rotor, PCV valve, charcoal canister, fuel filter, air filter, fuel pressure.
Connect a scan tool, look for codes, but especially look at the data stream. Assure that the O2 sensors are working properly and switch rich/lean quickly. Make sure the knock sensors are working right. Verify all the other sensors and outputs.
Reliability adders:
Send off an oil sample for lab analysis. Typically under $25. Consider sending a trans fluid sample also.
Drop the trans pan, change filter. Dump in fresh fluid. Drop a cooler tube, flush trans fluid (typically about 15 quarts, maybe more.)
Change axle grease, transfer case fluid, PS fluid and perhaps add a PS filter.
FLUSH BRAKE FLUID! Inspect brake hoses, consider pulling the calipers apart for cleaning and inspection; or just replace with "rebuilt" calipers. Assure the ABS is bled properly using a scan tool.
(Essentially, change all fluids and filters, including coolant.)
Verify brake linings and rotor/drum condition, and no hydraulic leaks. Assure park brake works and cables release properly.
Verify steering/suspension: ball joints (4), tie rod ends (4), control arm bushings (8), idler arm, Pitman arm, steering box rag joint (ultra-common problem on the vehicles that use a rag joint) steering damper (if used), shock absorbers (4) sway bar mount bushings and sway bar links. Check tire pressure including the spare, and make sure the spare tire winch and all the emergency jack tools actually work.
EVERYTHING made of rubber is suspect--tires, coolant hoses, brake hoses, PS hoses, trans cooler hoses (if used), accessory drive belt, control arm bushings, body-to-frame bushings, Air Conditioning O-rings, etc.
Clean mud, dust, debris, sparrows, crickets, etc. from the radiator, A/C condenser, PS cooler, external trans cooler, etc.
Put a flashlight with fresh batteries in the glove box, or behind the seat.
AFTER you have the basic "go, stop, steer" and other safety items inspected, you can think about "modifications" to improve performance