i have a 99 silverado with 185000+ miles on it, i was thinking of dropping in a 5.3 crate motor, will any year work and what mods will be needed?

 

4.8 and the 5.3 are basically the same motor. Expect a direct bolt in but you will need to update the pcm. Because the silverado had many differences in each engine options from drive by wire to sensor manuf changes, you will need to eyeball every hook up from the new motor to your current one to make sure they all use the same setup.

Expect a few speed bumps during tuning.