I still haven't giving up on HEI, but I'm also not quite sure how to to troubleshoot it/ fix. But I needed the car to run, so went and got a cheap Mopar ECU from O'Reilly's for now. Now the car runs without stalling again - woohoo! Unfortunately I had messed up the carburetor idle mixture settings - thinking that it was stalling because it was too lean or too rich. But I didn't really know how to, so in the end it was misfiring a lot. Then I watched a couple of youtube videos and realized that I had vacuum gauge that i found in one of my old cars. I wasn't quite sure what port to use on the BBD, but I have a port in the manifold for the a/c, so I hooked up the gauge there. I remembered that it was supposed to be around 15 and not fluctuate and I think I got it to a pretty good spot and now the car runs great again! It was never really set up and I had just bolted on that carb straight out of the box. I think there might still be some room for improvement, but the gauge was a great help and I will try to learn more.
Also, I realized the wiring is so bad that most of it needs to be redone. Yay, more work and no time, haha.
