A few little things, 1999-2003 1500 Series trucks are only available in Regular Cab/Long Box, Regular Cab/Short Box, Extended Cab/Standard Box and Extended Cab/Long Box configurations. GM didn't introduce a 1500 Crew Cab until the 2004 model year.
See page 17 of the 2004 Silverado Vehicle Information Kit from GM Heritage Center for reference.

2004 1500 Crew Cab [GMT800] shown below:


Not to be confused with the 2001-2003 and 2005-2007 1500HD Crew Cab [GMT880], note the 8-lug wheel and 1500HD badging.
2001-2002:

2003 and 2005:

2006-2007:


For 2004, there wasn't a 1500HD option, instead, those trucks were badged as 2500 (not to be confused with 2500HD :lol:) to avoid confusion with the newly introduced 1500 Crew Cab.


For 1999-2002, 2500 models were also available on Regular Cab/Long Box, Extended Cab/Standard Box and Extended Cab/Long Box.


Note the difference in grille and hood from the 2001-2002 2500HD shown below


Other differences to note are the mirror choices for the 2003-2005 and 2006-2007 2500HD and 3500 trucks.

2003-2005 offered the 'accordian' style power-extending mirrors on LS and LT trims (Chevy) and SLE and SLT trims (GMC).


2006-2007 models offered a new towing mirror, which did not power extend.


2001-2005 models equipped with the Duramax diesel engine use this badge:

2006-2007 models used this badging:


All 2006-2007 model Silverados had the 'CHEVROLET' badge on the right side of the tailgate deleted.

2002-2003 1500 Silverado with Extended Cab/Standard Box Fleetside configuration could be had with Chevrolet's Protec composite pickup box. This included a unique tailgate with Protec logo, as well as 'CHEVROLET' written across the tailgate in a similar fashion to the Avalanche. See page 13/29 of the 2002 Silverado brochure:
http://www.auto-brochures.com/makes/Chevrolet/Silverado/Chevrolet_US%20Silverado_2002.pdf

2002 was also the first year for GM/Delphi's Quadrasteer four-wheel steering system. Standard on Sierra Denali, optional on LS, LT, SLE and SLT models.
Note the wide rear fenders, roof-mounted clearance lamps and side marker lamps on the box.




2004 Silverado 2500 Crew Cab LT with Quadrasteer

A brochure of the 1999 Chevrolet truck range in Mexico, including the Silverado/Cheyenne.

http://storm.oldcarmanualproject.com/chevroletpickupstrucks1999.htm

The 400SS Sportside is interesting and badged as a 2500. Appears to just have the basic 5.3-liter Vortec V8 by the 255 horsepower output rating.

Nice find Truck_Guy! Finally can use the 10+ years of spanish to use