They will work, but it takes some doing. The IAC is different, so your harness has to be adapted and the computer re-programmed to make use of it. Also, you'll either need to open the ports in the intake manifold or use a tapered spacer so the blades don't bind on the intake manifold. Fuel pressure regulator and injectors are the same style, so it's not difficult to get the fuel pressure adjusted correctly and to pop in the proper sized injectors. It's relatively simple, but not worth it on a 4.3L. There is just no need for that much air flow. You'd be better off just doing some minor modifications to your stock TBI unit.
Space up the injector pod, remove the 1/4" lip around the venturis and switch out to a regular air cleaner, removing the 1" spacer-majig and you'd be surprised how much more air will make it's way through.
ya i knew i would have to get an adapter plate i just didnt know wat else i would have to do to make it work cuz the motor in the truck is a 4.3 thottle body rite nw and i got a vortec 350 and i also got a 454 thottle body layin around i was jw if i could make it work sence the truck is set up for tbi already
You can make it work as I stated above, it's just really not easy. If you can have the downtime to do some minor mods to your TBI unit, I would suggest using that over the 454 TBI just so you don't have to screw with the tuning so much.
For a Vortec 350 you have a few options for the intake manifold. You could get GMPP #12496821 that will not require an adapter plate and will have EGR provisions built in. This is technically the easiest and "right" way to do an emissions legal TBI vortec engine. It's $410. The next option is GMPP #12496820, it is a four barrel spread bore carb intake with EGR, you will need an adapter plate, but it technically should pass emissions when done, it's $315. Beyond that, any dual plane carb intake should work just fine, the smaller and closer to stock the less tuning will be required, but with vortec heads and TBI, you're going to need a custom chip, bare minimum.