What's a good way to test this whole system. I think I've read in the past that it will kill mileage if it's not operating correctly, and I'm currently running out of things to check. The drop in mileage came on quite suddenly.

Fuel pressure good/holds, etc
Timing is at 10
Air filter clean
EGR... well, blocked off since the port is broke off.
Battery terminals clean
Plugs, wires, etc fairly new (will check here in a minute)
Alignment good
*#1 plug looked ok I guess.
*TPS voltage and range ok.

Actually, the plug has more 'white' than anything, combined with engine vibration, it seems a lot like there's a vac leak. I'll try some spraying some stuff around or see if I can find it using some smoke.

There's 275k on this thing, so I pretty much expect anything and everything to start going bad.

 

Don't know if this helps. Looks like you did the 3.4L swap. I just did that on my 92 Camaro. I am in Denver right now (thin air) but brought the car from South Bend here at Xmas and the mileage was only about 16mpg at best after I got here.. I did a cold air intake change which brought my mileage up by 4.6mpg... I replaced the stock intake with that very small snorkel with a dual air intake from the 5.7L Z28... That changes the cross sectional area of the intake from 1.07 square inches to 7.07 square inches. My power came up a bit but I think I need an ECM tune now to bring it any higher mostly due to the altitude here.