I recommend getting the front and rear bars together. Best overall improvement. The rear only would work OK, but the pair transforms the car.
Most of the parts numbers on these pages are outdated, I think, but your Ford parts guy should be able to get you the right stuff in just a few minutes:
http://bradbarnett.net/service_man/C...uspension/204-02_Rear_Suspension/REMOVAL_AND_INSTALLATION/Rear_Suspension_Exploded_View.pdf
OR
http://tinyurl.com/22puvo
You'll need Numbers 1, 2, 3, and 6 on page two. Everything else should be in the "kit". It's possible to do it with hardware store parts, but genuine is safer.
Edit: No problem with a properly installed dual exhaust: both should be symmetrical.
Edit: add tiny URL of the PDF pages that got folded somehow.