[Rev]

Yes you can certainly install headers with the engine in the car. Some are probably easier than others. I remove mine every so often to replace header gaskets to fix exhaust leaks. Not a big deal for my Tri-Y headers.

You will probably need a muffler shop to make the head pipes connect to the headers collectors (bending and welding). I'm thinking long tubes here. Again, not a big deal, but you would have to get the car to the shop without the pipes connected to the headers. Very loud right straight out of the headers. Almost sure to get you a ticket if you actually drove it on the street to get to the shop. I had mine towed to the shop for a custom made 2 1/2" system with a cross over "H" head pipe.

Rev