it can all be up graded to a 99.5...the only thing early 99 on my truck now is the build date..here are some of the differnces..
Early 99 has a wicked wheel in it from the factory.
The early99 has 120cc(AB code) injectors and the late99-03 has 140cc(AD code) injectors.
The early99 has 2" intake runners. The late99-03 has 3" intake runners.
The early99 has a 15* swash plate HPOP
. The late99-03 has a 17* swash plate HPOP
The early 99 has a 110v IDM
..the 99.5-03 have a 120v IDM
The early99 has one glow plug relay. The late99-03 has one glow plug relay and one air intake heater relay side by side. Both relays are on the passenger side valve cover toward the front of the truck.
The early99 has a totally different air filter that is not inter changeable with the late99-03. But you can put the late99 air cleaner assembly in the early99 truck. The only three aftermarket air filters that work on the early99 are the AFE stage 2, the AIS and the KWIKK 6637.
The early99 track rod has smaller mounting bolts than the late99-03.
The early99 has shallower rotors than the late99-03.
The early99 has shorter hub-bearings than the late99-03.
The early99 front axle U bolts are square where they go over the spring pack. The late99 U bolts are round.
The early99 has a different pitman arm than the late99-03.
The early99 does not have the AIH
like the late99-03.
The early99 does not have the automatic transmission fluid cooler in the radiator.
The early99 has no indicator light showing that "cruise" is on. Late99-03 has a cruise light that will come on when you set the cruise control. The early99 truck has the light in the dash already. A newer PCM
(Yes it will plug in and will run better too) must be used to make the cruise light illuminate. It is a green light on the right side edge, half way up.