In a fullsize GM truck, the L35 4.3 would only be more fuel effecient on the highway MAYBE.
200hp/260 lb/ft torque for the L35 4.3
230hp/285 lb/ft torque for the L30 305
I know that both use the single cat, y pipe arrangement with 3 precat O2 sensors and 1 aftercat O2 sensor.
The PCM's are the same, just need to be reflashed with the proper V6 calibration.
Be sure to use a V6 TC or the truck will be more of a dog. The V6 uses teh DBLF higher stall TC than the V8's.
You would need a V6 fan shroud. Trans will work, driveshaft is the same.
The V8 harness will work if you swap out the big connector that plugs into the fuel distribution block on top of the intake manifold. MAF is the same,
Throttle body is different between V8 and V6.
But honestly, I would find another 305. The 4.8 is a good idea, but off idle torque is lacking, but they pull great after 4000rpm.