I've researched this when I was building my RS clone. As I was assembling the suspension and chassis components I'd take comparison measurements. As I was using a SVT/ST170 as a starting point, that will be my reference figures. Start with the wheels and tires, stock 7inch wide compared to 8inch, there's 13mm, half of the 1inch with of rim. Now that width is 7mm farther out, the difference in offset, stock 49et and RS's 42et. Then on the front is spindle length and on the rear is spindle length, where I find the rest which is around 12-13mm. Add it all up to total 32-33 times 2 for both sides you have 65mm increase track.
Rim et.............49-42= 7mm
Wheel width 7>8-half=13mm
Spindle length=.....12-13mm
Total per side.......32-33mm Total for car the 65mm claimed