Is the 5.3 an LS engine?

Yes. GM's 5.3 L Vortec V8s from 1999 onward are part of the LS-based small-block family (Gen III and Gen IV): LM7, L59, LM4, L33 (Gen III) and LH6, LS4, LY5, LC9, LMG, LH8, LMF, LH9 (Gen IV). Gen V 5.3s (L83, L8B, L82, L84, 2014+) are the newer LT-family direct-injection engines, which use different parts and electronics.