2460 y pipe size and fitment
 

So i have these headers installed and when going to get an estimate from a muffler shop about a custom y-pipe they told me 3 inch would not fit... Is this true?? stock 89 Trans am GTA with exception of headers.... shop said the pipe would be to large to fit up under and between the frame???? mind you they have not looked at the car yet....

Baloney. The Hooker 2055HKR Y-pipe is 3" OD. Aftermarket '86-'90 TPI single direct-fit cats are 3".

The 16767HKR Y-pipe for the 2460HKR headers is only 2-1/2" OD (and eliminates the cat). If you have any kind of power at all, you'll want 3" after the downpipes join.

You could do dual 2-1/2" cats, merged to a single 3" I-pipe.