As I said, it's been ~20 years...but I don't remember that at all.My 88 was a little different, it had a ball joint with no gaskets. My headers are the same. The Y pipe is 2-1/4" to 3".
Might it be that a 2 1/4" pipe would be flared-out to take a 2 1/2" donut? I'll have to break out the tape measure and verify. Yes, I'm fairly sure the catalyst-end of the Y-pipe I have is 3", because that's what I bought for a catalyst, and muffler--3" inlet, 3" outlet.
Connect a scan tool, look at the data stream.It runs but cold idle is bad it goes up and down almost stalls. When warm seems good. Any idea? Map or iac?
What is the actual fuel pressure under load?
How long has it been since the vehicle got new plugs/wires/cap/rotor/PCV valve/ignition timing verification, air filter, and throttle-body cleaning?