wonder if the fuel pump relay has arcing damage from years creating an internal resistance

 

You have a Vortec?

Whole different injection system with the poppet tips.
I would start to inspect those.
From what you say, your fuel delivery to the spider injector appears perfect.

 

Check to make sure the o-ring between the throttle body and the air breather is installed correctly and/or not damaged or missing.. If it's not correct or is missing it screws with the MAF readings. Basically it's a vacuum leak and can cause long cranking issues. Shows up as negative numbers when you test your long term fuel trims.