8" should be plenty, but be careful not to extend them much more then necesary. If you extend them to much you will get an SES light saying O2 sensor slow response, this happened to me and it made the car run like poo.
call Randy at universal ford in baltimore. Got my brand (best kind out there) new 02 sensors with long extensions. $45 bucks or so a piece but WELL worth it especially if you 02 sensors are old. Replace them while your under there.
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