For whatever it is worth, I took several measurements. The rear springs and tank are out of my 69 pace car. Measuring inboard edge to inboard edge, using a stanley fatmax tape measure with bushings and a bolt in place to get the centerline, I came up with 42 1/2". Went right to left, left to right, several times to eliminate any possible error on my part. I also did this on my 69 x22 coupe drag car, which still has factory rails, but has coil overs and a fuel cell in the trunk. I came up with 42 9/16". Both cars have the original rails. Hope this helps.