I took OEM off my brand new truck in '02 and have run the Bilsteins ever since. Great match. The drilled air box is/was a very common mod for more air flow and letting out any water ingested, but tended to dirty filters faster due to changed inlet. Don't use foam, use good AC delco paper filters. Every objective test ever performed has shown no appreciable improvement even with fresh perfect oiling, and reduction in flows otherwise, plus MUCH higher abrasive silicates allowed into the intake, not to mention potential oil fouling of the sensor. Also check each and every quick connect, particularly those on the heater core lines. They get brittle and fail catastrophically with little or no warning. Also, the filter head and short fuel lines connected are both prone to air leaks. They cause hard starts and loss of prime, often with no outer signs of issues. Add in a tuner like my Juice/Attitude and you've got a very nice well rounded truck. The stock stainless exhaust is very good for 100-200 hp, the only real focus would be a free flowing muffler. The restrictive exhaust manifold upgrade and larger pipes only if you really turn it up and/or tow hard (EGT issues), but then you'll need to be looking for an upgraded TC with potential Trans upgrades too. There is also a common issue with (IIRC) the intermediate shaft in the trcase, google can provide specifics. Other than that, I can't think of anything else...