Review this link,
cold air intake
open up the air box, and clean up front first. With those miles, the likelihood of buildup in the Cooling assemblies is great.
Cleaning up front, DO NOT use high pressure, as the danger of damaging cooling fins in the elements is too great. Washing from the front is not going to penetrate through and past all of the other elements in front of the radiator for cleaning anyway.
You need to pull the cap across the top of the radiator support in order to clean in between all of the elements, removing the bugs and debris collected from over time. A garden hose with light pressure is all you need.
Get ‘er clean and breathing first, Rule out any buildup in the lower radiator core, then go from there on mods.
On edit: When it's time to service the cooling system, (flush, new fluid) one should always address the buildup up front, but most places don't.
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2005 Chev Duramax/Allison 3500LS DRW Crew Cab
CAI / LMM intake OH kit, Sulastic springs, Bilsteins,
B&W 30K-Turnover GN, Putnam XDR 15K, Ride-Rite Air Springs, A/T Revos.
Pulling 10 tons daily.
2006 Chev 6.0L/4L80e 3500WT DRW Ex-Cab
9 'Flat/GN, Ride-Rite Air Springs, A/T Revos.
Pulling 6 tons daily.
"The difference between liberals and cannibals, is that cannibals eat only their enemies." -Lyndon B.Johnson 
100th