I've got a '91 camaro, removed smog and a/c so passenger side is pretty open. I've got hedman shorties with 3" collectors already not installed yet. Would I be better off with these or go long tubes? Will there be room to fab a 3" y pipe from the shorties down to the cat?

 

There's plenty of room for shorties and a Y-pipe. But if you go with long tubes, you're probably going to have serious ground clearance issues, though they will give you more power.

I would stick with the shorties.