Yeah, that's exactly what I want to do is have a cat fabbed to the correct length I need with the flanges welded on so I can just bolt it up.
I'm having trouble finding a good muffler shop though that does decent work. The last place I went to, to have my stock exhaust welded to my header, butchered it and it's all kinds of sloppy and uneven etc. You should see what the collector coming off my nice JDM header looks like now....ugggghhh is all I got to say about that

Consequently I'm also now looking to either replace my header or get a new down pipe for it... one with a collector & flange that's not all butchered up like mine is now.