I just bought a long tube header kit and an X-pipe for a 1999 Corvette C5. My question is: do I need to buy an extension for the oxygen sensor wiring, or can I use the original one?
This is the kit

 

Welcome to the forums. Nice pipes. They usually recommend extension wiring sometimes, but it depends on where the sensors are now. To be on the safe side you could buy a set off amazon or somewhere for standby and if you don't use them you can send them back. There might be enough slack in the original harness to work, harness vary alot sometimes. You might want to peek under and see where the sensors are now. I would think you could vary 6in one way or the other depending on the harness. If you are having someone doing the pipes to be on the safe side, so you don't wait for parts.