Not a lot of performance parts were made for the 4.3 to begin with.
Changed a few things around on mine with a Hyper-Tech tuner. All that did was raise my shift points 500 rpm & re-calibrated the computer to compensate for the larger wheels. Not a lot of performance to be found there.
Changed the exhaust to a single 3" cat back. That probably hurt the performance! It'll work fine for whenever it does get a V8.
I looked at headers. Never could find any used ones. New ones? Yikes! I could buy 2.5 pairs for an SBC for the same price!
Then, I ditched that %$#@! gov-lock, added a 'real' posi (Eaton) & changed the gears to 3.73 from the 3.42's.
Holy schnappin' areseholes, Batman! That woke it up! Suddenly the truck had 'some' performance! 4.3's aren't supposed to turn on like that! Big improvement on the fun factor!
But...the gears killed the gas mileage. Or maybe I was just having too much fun! Hehe!
If the intake manifold gaskets hadn't failed, which was the demise of that engine, the next trick was a real 'tune'.
Along with the marine intake, that's about all that I found could be done to 'The Mighty V6' with bolt-ons, short of building an engine from the crank, up. Which CAN be done, but at an exorbitant expense! I can have both my trucks 6.0 powered for the same $$$, & be packin' 2X as H.P., each, with much more tuneability via a 411 swap & EFIlive!
4.3's were built for budget minded consumers. They do get fairly decent mileage, but a 5.0 or a 5.7 in the same vehicle can achieve almost the same mileage, with better performance. And there is a huge aftermarket for anything SBC.
Not saying to swap in a V8 right now, though. Just drive that little 4.3 till it drops, then do a swap to an 8.