Decat would be better if the car has an aftermarket exhaust as the piping would be the same diameter, but there's nothing wrong with a gutted cat, free mod FTW
The nut should be loose so you can easily adjust the arm. I'd take the FMIC off to have better access, last thing you want to do it round the bolt off! Take off the Actuator and soak the nut in silicone spray? That might help loosen it as WD40 evaporates relatively quickly
You're more likely to lose the clip if anything, they are an awkward bastard of a thing! lol You measure the distance of the arm then adjust the nut and turn the arm to the right length http://dub-se7en.toyotagtturbo.com/useful_...hksactuator.pdf