Hello I'm new on here ! My friend has a 2000 GMC with 5.3 . Has the whipple sidemount . He said he would sell it to me for $1,000 for all of it ! Is this a good deal ? And would it work on my 2004 chevy 5.3 ?

Physically it would bolt up, the issue is the electronics. Those two trucks have different PCMs.
Depending on your level of motivation, it's possible to install larger injectors in your stock intake, a 2-bar MAP sensor, and reprogram your PCM for compatibility with the new sensor. That would allow the omission of Whipple's auxiliary fuel injectors and associated electronics.
Some owners do this modification to existing working systems. It should be noted that recalibrating/reprogramming the PCM isn't technically smog-legal, depending on where you live...

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"James B." '99LT_Tahoe4Dr2WD, Tuned L29 Vortec 454, 62#Injectors, Cutouts, 4L80-E 10.5"14bolt ,EatonPosi-4.56:1, Gear_Vendors, HydroBoost, (NEXT: Brodix heads, 230/240 cam, SS roller rockers, longtubes, THEN: Intercooled W200R-8psi in the future...)