I never thought of measuring the rear end of my build until it was too late. Basically, anyone using the Miata subframe has to consider the fact that they will be fabbing the entire back end themselves. I just built what looked right; I saw some pictures of Mark Rivera's build, and the Rorty plans, and from looking at those, I plowed on, simply measuring to make sure everything was identical on both sides.
One thing you might want to consider is using hard differential bushings--I bought an steel set from Rivera after seeing them on his photo site.
http://www.locostusa.com/forums/posting ... ply&t=3843
http://kit-car.org/Files/suspension/Ror ... sembly.pdf
Although the wheelbase might extend a little (Mine is 97.5", if I remember correctly), I think the overall length is about the same, as I have a pretty short back end.