Hopefully the issue is corrected now.
I pulled the intake plenum this morning and saw this:
I dug out the MPFI parts and finally did the swap. MPFI next to CSFI spider.
Here's a closeup of a 'good' poppet - one from a cylinder that didn't have a sooty spark plug:
Here is a 'bad' poppet. Can't really tell the difference.
I see some clean spots on the injector retainer plate and on the regulator housing clip, so it looks like that's where it was leaking from. I didn't give it a great deal of attention as I wasn't looking to repair it.
Regulator on the new spider, for comparison.
Cleaned up the lower intake manifold a bit before installing the new bracket and fuel meter assembly.
The GM TSB recommended routing the #3 injector line behind the #1 to avoid interference with the upper intake plenum. Those hoses are a bit fiddly to get them right where they need to be. Fortunately they seem to bend more than you think they will.
Started up on the first try and puffed out a bit of smoke - burning off all the carb cleaner I used! After that the idle settled down. No shaking, no audible misfires, no codes. I took it on a test drive on our back road and then on the highway. Running a lot smoother than I can ever remember - idle quality is better than what I've considered 'normal'. Went through an OBD II drive cycle - kind of a pain to find a stretch of road where you can coast from 55 down to 20. No codes yet. Also, the smell of unburned gas in the exhaust is gone - just smells like hot air as it used it.
It feels a bit sluggish in the low end, I'm wondering if it's just the bad plugs as I haven't changed them yet. I picked up new ones last night but this was enough work for one day, I'll swap them tomorrow morning and see how it goes.