When I did my L31 into my 98 ZR2 I was surprised to find that the original 4.3 fuel lines actually bolt up correctly, right to the 5.7 SCPFI control pod on top of the intake. I suspect GM designed them that way so as to simplify things on the truck and van assembly lines by being able to put in an L35 or a L31 w/o any changes. At least to the fuel system.
Unfortunately, I'm under the impression you'll need the complete fuel system from a 4.3. IIRC all the way bcak to and including the pump assembly. I could be wrong, but I thought that even the pumps were different. Sounds like a trip to pick a part is in your future. If you can find a 4.3 truck with the engine already out shouldn't to bad a job just removing it all. As long as they aren't too rusty. Would be even better if the bed was off.