07 silverado 1.5" body lift
 

If I were to install a 1.5" body lift would I need to extend anything?

Nope just install and go. Only spot that gets tight is the e-brake wire that runs along the drive side. Follow the instructions and you're good to go

Have you ever installed a body lift? Not being wise. Just want to know the difficulty of the installation.

I've installed 4 and gonna do the 5th on my buddies truck soon. All 1.5" body lifts. They're pretty easy, the bumpers take the longest gettin lined up right