Welcome to the Ford Mustang forum built for owners of the Mustang GT350, BOSS 302, GT500, and all other S550, S197, SN95, Fox Body and older Mustangs set up for open track days, road racing, and/or autocross. Join our forum, interact with others, share your build, and help us strengthen this community!
FWIW I'm pretty sure a lot of unibody cars are off. A quick way to measure wheelbase from side to side is to latch the tape on the rear of the front wheel rim at the back, then measure to the rear wheel rim at the rear and this will give you a quick side to side comparison.
In any case, trailing arms should always have some form of adjustment. This is part of the process of "stringing " a car, basically measuring from known points that denote the center of the car on the front and back to the outside edge of the car. (I'll try to find a pic of it)
Remember when I said we could learn a lot from the dirt sprint car guys? Well they actually set toe from those strings since they occasionally run offset wheelbases.
Here, a perfect explanation http://www.hotrod.com/articles/ctrp-1204-determining-wheel-alignment-string-your-car/
Sound like what you need to do is install all of your adjustable stuff, then pay the owner of the alignment rack to come in on a sunday and spend the day aligning the car.
Once that is correct, mark the center of the car and get some fishing reels with some bob weights and write down those measurements so you can repeat them at the track w/o the alignment rack.