[THEFERMANATOR]

Since you say the problem persisted after PCM replacement, about the only thing that comes to mind to me is a bad ground. The early SBEC is pretty basic, but normally reliable. They can be shorted out and do funky things if the crank sensor gets shorted out, but I've never heard of one firing the injectors and coil at the same time. So either you have a bad ground allowing the board to backfeed somewhere, or you got 2 bad PCM's in a row. The only other thing I could think of is a short to power on one of the sensors freaking out the PCM. I can honestly say I have never heard of a single case like this before though. The most common problems on those early trucks was shorted out crank sensors causing a no start.