If you have the Revell Escalade, that would be the easiest to add. Just cut off the Silverado's mounting brackets and add a strip of 0.040 thick styrene to the bottom of the chassis to raise the suspension up to the correct ride height.
The picture below shows what I have been looking at to use in my builds. Bottom left: AMT Escalade EXT mounting bracket is a little wider than the Silverado's frame. Top left: Die-Cast Promotions' Chevrolet Express van mounting bracket, is not as wide as the Silverado's frame. Top Right: Matchbox 1/24 scale 1999 Silverado 2500 front suspension, definitely larger than the Silverado's kit. Bottom Right: Revell Escalade front mounting bracket, fits just about right.
I'm experimenting with modifications to the Silverado front suspension for future builds and have begun modifying that kit suspension parts to save on chassis modifications. I plan to swap in the Revell Escalade front axle on the 3500's I'm building since it will accept the metal axle tires from the AMT Chevrolet 3500 snap kit.
As a side note, the transfer cases are molded to the chassis for both Escalade kits, as well as the Express and Suburban. If you don't have any spares, shoot me a pm and I might have an extra or two laying around after this weekend.