So I've just finished putting this engine together, a 2004 Vortec 350 from GM industrial engines:
906 Vortec heads
Comp 981 springs
1997 F car LT1 cam
Ceramic shorties
GMPP TBI intake
1992 large footprint 454 TBI with 17084304 injectors at 13 PSI
1" two-hole TBI spacer (to seal the Iac passages up)
EBL Flash
http://s892.photobucket.com/albums/a...-35-45_326.mp4

So, despite plugging every vacuum port but the Pcv and map, shoving a plastic bag into the IAC port to seal it off, and backing the idle screw out until it doesn't touch the throttle lever, and having less than 20 degrees of timing at idle, this thing doesn't want to run at less than 800 rpm.

I exhausted a can of starting fluid trying unsuccessfully to find vacuum leaks, checked the torque on the TBI mounting bolts (12ftlb)

 

Used felpro permadry 98000 gaskets and ARP bolts to secure intake, doesn't seem to be any kind of obvious vacuum leak. Doesn't seem to be sucking in any oil like an intake runner leak would.
It's making 21" at idle right now un-tuned.
It seems to be idling very smooth

I cannot figure why the engine won't slow down.

 

Your PCV is a regulated vacuum leak, you may be getting too much for your setup.

 

That's a good point, I'll be sure to check it...

Have you (or anybody, I guess) ever seen PCV cause such a behavior?

 

May have an incorrect TB base gasket? Make sure it matches the base including IAC passageway that it seals. I fould this out since I matched up the aftermarket gasket with base befiore installing and the gasket did not match. the Delco gasket did.

 

sanity check-
when you short out the ALDL and the ECM runs the IAC down to zero counts, should the plunger be run all the way in (towards the IAC motor body) or should it run out (away from the IAC motor)????

 

It should be fully extended zero steps. That procedure is also used for setting min air for idle.