Is your mechanical ability up to ***** for doing an LS swap? You need to do a ton of research, look into what parts others are running, you need the following for a "basic" v6-v8 swap and that is not the 4.3L to LS list. No, you won't need to swap your rear end out.
V8 frame side mounts
V8 engine side mounts and LS adapters(Dirty Dingo or ICT Billet)
Transmission Mount
ICT flexplate kit(will eliminate the spacer need)
Plug and Play harness(PSI in your case)
0411 ECM or 058 ECM(depending on year of donor vehicle)
TAC module(again depends on year of donor)
Fuel pump that supports your end goal(not your current)
V8 Radiator(34" wide)
Camshaft of your choosing
Valve Springs
Gasket kit(engine gasket kit from Fel-Pro)
A GOOD TUNE!!! (THIS IS THE MOST IMPORTANT THING!!!)
There is a lot more to this list, but that is what I can think of off the top of my head. There is a ton of us doing LS swaps on the site,
@Wh4t3v3rs,
@0xDEADBEEF and myself to name a few, there are others. If i remember correctly
@0xDEADBEEF went from v6 to LS