We're having an issue with an LS3 we've swapped into a 72 Chevelle. Using modified stock harness, stock ECU (from 2010 GMC Truck/6.2). I've turned off both VATS1 and VATS2, disabled starter checks. Car will start, run for maybe 1-2 seconds and dies. I'm also not seeing any RPM signal when using VCM scanner (along with a speed of 158mph). I've attached the tune, it's completely stock other than the vats/starter mods. Car has long tubes and a holley EFI intake. I originally thought the intake could be causing it, but the lack of RPM in scanner makes me think something else is going on. Ideas? Tune attached....