I haven't heard of a c5 sensor although I ran into the same problem you did. The best option I found was to plug the o2 bung the headers came with and weld in a new bung a little further down. If you don't know how to weld most exaust shops will weld in a bung for about 40$ which is probably cheaper then the sensor your talking about. Idk how big the c5 sensors are but, even if they were an inch shorter id still be nervous about what happens when the motor is jerking around during hard shifts and going WOT that it still might hit the firewall.
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