Before ordering the exhaust system, you might try checking with a few local muffler shops to see what they would charge for installing a system. Most muffler shops have tube benders to custom bend a system and make it fit right and connect to your existing mufflers or any other mufflers you want to use. Shops will also use a thicker wall pipe than what you get in the kits.
A shop will also weld in an O2 bung; it's a good idea to buy a new O2 sensor as most old sensors will have rusted themselves in place after the years.
Try to find a small independent shop. It's technically a violation of federal law to remove existing working cats from a car and a shop can be fined for doing that if they get caught. It's in the 5 figure range
But you can remove the old exhaust yourself, put the car on a trailer (or drive it), remove the license plates for the day and take the car to the shop. Then you tell them it's "for off-road use only".
Ask them to add an X-pipe in the spot where the main cat was located. That will reduce interior resonance and provide a better sound note. There will be no codes from removing the cats or leaving the one air pipe that went to the main cat open. But the O2 sensor has to be connected and working otherwise that will set a code.