VS sensor help

Hello there. I need some help working on my car here. Its a 1983 Grand Prix. It originally came with a 305 4 bbl. I swapped that out for a 1989 350 TBI out of a 1989 Sierra. I installed all of the sensors and the ECU, but recently I am having a dilemma. The car started running like absolute crap, whenever I accelerate with half or more throttle it backfires and usually stalls. I hooked up our scan tool to the ALDL and checked for codes and the VSS code came up. This makes sense as I have no vehicle speed sensor. I think this is causing it to give false info to the ECU and leaning it out and changing the timing. The car came with cruise control so I am wondering if the old speed sensor for the cruise can be adapted to work as a VSS.

As a temporary fix, I plugged a temp sensor in and let it sit on the manifold to richen up the mixture, but I gotta figure this out, its nowhere near the performance it should be with the mods I have done to it.

-Thanks for you help. Nick.