An LS3 has just about infinite options these days. You can buy a crate 430hp or a crate 525hp engine. GMPP makes a factory oem computer and wiring harness that fits the engine hp. You can also easily buy the accessory set-up depending what you want as far as p/s or even a/c. If you only want the alternator, they make that one also.
If you go on
CrateEngineDepot.com they have complete kits with engine, trans, and computer to drop right in. If you want more hp later, there are about 500 more ways to get an extra 100hp pretty cheap.
Oh..and witht he engine being all aluminum, they are pretty light weight and compact.