It doesn't matter what the torque spec is for that center bolt because you can't measure it anyways. When tightening or loosening that bolt you have to hold the shaft from spinning. You can't get a conventional torque wrench on it anyways so just tight like the German engineers do, goodentight. Yes you can use an impact gun on it. I did. Don't mess with the 4 little strut mount bolts unless you are changing out the strut mount also. Which, by the way, I think the Ford service manual says to do whenever changing struts or springs, but hardly any body does.
The easiest way to change your front springs: Put front end up on jack stands. Undo/loosen up the brake line brackets and all of that stuff. Then put a jack under the A-arm and jack it up to support the suspension. Undo that center bolt and slowly let the jack down and the strut and spring will clear the mount. Undo bottom strut bolts and remove the strut & spring. Replace spring & strut, bolt uo bottom bolts to 148 ft/lbs, and then proceed to jack the whole assembly back up into the strut mount tighten that center bolt and your done. Well button everything back up. Just remember to line up the end of the coil spring to the upper mount as your jacking it in place.
By the way do you have camber bolts?
The rear upper shock mounts are the same deal with the spinning shaft. Just take a look at the rubber bushing and how compacted it is and then when you replace the shocks. Tighten down that bolt until you get the bushing to compress to the same height and consider that torqued to spec. Torque the bottom bolts to 85 ft/lbs.
If you really want to know, the front center bolt should be torqued to 46 ft/lbs, and the rear upper bolt should be torqued to 30 ft/lbs.