It's the small things that add up doing the 4.3 to V8 swap. I'm a few weeks out on doing this same swap on my 2000 Sierra. To keep cost down look for a good engine in the 99-02 year range. If you try to use an 03+ engine you'll have to find a 99-02 wiring harness and PCM, return type fuel rail, cable throttle body, 99-02 oil pressure sensor, a/c compressor with the mounted high pressure cutoff switch.

Finding a "drop out" engine with all the accessory drive brackets, tensioners, idlers, A/C compressor and starter saves a lot of money and parts scrounging. The alternator and power steering are about the only things from the 4.3 that swap over.

You will need a V8 upper and lower radiator shroud, V8 mount bracket(s), fuel lines, V8 throttle cable bracket, branch heater hose, 2nd heater hose, new heater hose plastic quick connects, V8 air cleaner intake duct (tube), V8 a/c accumulator, V8 a/c discharge hose, V8 upper & lower radiator hoses, new belts. Probably want new wires, plugs, valve cover gaskets etc.