Can I swap out my 92 c1500 4.3l vortec with a 85 monte Carlo 4.3l v6 engine

 

Sure why not. I would swap the ECM over as well. '92 should be a 16146299 which is a pin for pin swap aka plug n play for your Monte's ECM or a 1227747. That vortec label on the engine is kinda a misnomer, it's not the sought after higher flowing vortec heads but I believe it to be typical TBI swirl port heads. Be mindful that if you use the serpentine accessories from the truck, a/c and alternator swap positions so you need to address that perhaps getting a Regal/Cutlass condenser with passenger side fittings. Also the water pump is reverse rotation on the serpentine setup so you need to keep them as a set.

Edit: Oh crap am I reading it wrong...you want to put the Monte engine into the truck??? You could but you'd be down 40-50hp.