Plug N Play Flex Fuel kit for 99-07 classic GM trucks using a factory style intake manifold and fuel rail combination. This will NOT work if you are running aftermarket rails. The kit includes a Plug N Play harness that connects to one of your ignition coil (whichever one you choose) and has a single wire that gets pinned to the PCM.
This kit will work if you are running a NNBS / TBSS intake manifold  and rails on your NBS as well.