I've been looking for info that would help me figure this out and I surrender- I'm hoping someone has run across this.
My 89 GTA came with a new Flowmaster mandrel bent system, complete from the stock manifolds back- with a test pipe in place of the cat- but the flange is there, so the system was made to take a stock style cat.
I have to put the cat on it and remove the test pipe, so one is on the way right now. Hope it fits.
Now, the odd part- it seems a little odd there would even be a nice mandrel bent y-pipe meant for stock manifolds, but it is what it is. Is there a header that will work with that y-pipe? Alternatively, is there a header/y-pipe combo that will work with the stock 3" cat in the stock location?
Fabricating a y-pipe isn't going to happen. I'm not that good at welding and honestly wouldn't want to spend that much time on it.

When you buy a header kit they come with the y-pipe. And your cat/filler pipe will mount to the new y-pipe once the correct flange is welded to it.

It seems that most of the y-pipes I've seen advertised never mention if the stock cat in the stock location can be retained. They seem to assume that everyone is having a shop cut, weld and bend pipes- whereas I'm looking for bolt-in. There just aren't any shops in my area with competency approaching anywhere near what it takes to get me to pay for their work.

You already indicated your current set up is flanged for the the cat. The header ypipe is a slip fit set up with a clamp like the stock one. So any ypipe you purchase with need to have a flange welded to it to work with your cat/filler pipe

it seems a little odd there would even be a nice mandrel bent y-pipe meant for stock manifolds

There were a few. SLP was one company.
https://www.thirdgen.org/forums/exha...-question.html

the stock cat in the stock location can be retained

That is exactly the conditions that the aftermarket had to meet.

Hopefully, your new cat will just bolt in place of the test pipe.
Short of going to headers, if you have a stock, or near stock engine, and the high flow
TPI manifolds and a full 3" pipe to the muffler, you do not require anything more than
you already have.
What does Vermont require for emissions?

OK, cool- so if changing manifolds to headers, the y-pipes should end up lining up with the rest of my system if I'm understanding this. I have the stock high flow manifolds going right into the aftermarket large diameter pipes.
Not sure if headers are going to do much, but keeping options open.

I'm relatively sure the new direct fit cat should bolt right in- the flange is there and the length of the test pipe measures out to the same as the original cat.
Vermont inspections require no 'check engine' errors lights but doesn't scan pre-OBDII vehicles... just visual. No light, all emissions equipment it came with must be there.
Lots of people running catless and getting their buddies to hook them up, but although retired- I was in the automotive business the last 10 years or so and need to keep up my reputation so I can get gigs as a professional witness in court or for state government transportation committee testimony. Gotta be 100% legit.
I know late model stuff inside and out, but still learning the older ones. I graduated in 88 and lusted after the GTA back then... kinda cool to have one now as a toy.

...graduated in '88 and now retired??? Did you hit the lottery?

Only part way- managed to put some money away, but mostly because I was in a helicopter crash in the military and am permanently disabled.

Oh, that sucks. Thanks for your service.
Glad you can still enjoy working on cars.