Upgraded to the new spider and the truck runs much better. Seems to idle higher initially, but calms down after a little while. Still losing 10 psi of fuel pressure or so within a minute after pump shutdown, so I'm getting myself mentally prepared to have to buy and replace the pump soon. Thanks for the help everyone.

 

Could be the regulator, injector(s), or the pump leaking down. The only way to determine for sure is by isolating the pump just after it pressurizes the system. The new injector assembly also came with a regulator, so if it's still leaking like it did before, most likely it's the pump, (as Kyle mentioned in the first response). Use AC Delco or Delphi pump only, don't go el cheapo.