Mine is for an Mx-6 and it fit perfectly. As for install jack up the back end to get the tires off the ground, so when you let it down the top of the struts will put pressure on the bar. The result that I noticed was more turn in and the back finally felt like it was doing someting instead of just going along for the ride.
I originally installed the bar without jacking the rear end up. I then read that some people said you should jack up the rear and re-install the rear strut tower bar. But honestly, afterwards it made zero difference in my P5.