I’d like to put a new engine in my 2000 Silverado 1500 with 4wd to make it
better for off-roading and possible drag racing. Any ideas on what I should
Those are two completely different builds. You're best bet is to join an off roading forum or performance forum and figure out just what you want to build there. As long as you have the $$$ for it, cause this is an expensive hobby.
Gears are the thing for offroading unless you are building a mud bog truck or something for the dunes in which case you need maximum power.