A few things to think about; make sure the frame is square to start with. On a Ford like this, in stock config, the spring center holes on the cross members would be the best reference for wheelbase, but that no longer works on yours other than as a general reference against original measurements from a drawing such as Wescott posts online: http://www.wescottsauto.com/WebCatal...ram1935-40.pdf Keep in mind, as the new semi elliptic springs in the rear are loaded they will flatten out extending the wheelbase slightly (how much depends on arch, spring rate, how much load).

If your MII front end wasn't a hack piece to start with, and if it was installed correctly, the front axle center-line should be in line with the upper shock mount holes (once the rails are square to each other that can be checked for square in relation to them). Then you can calculate/measure to the rear axle center-line. This is where it gets a little busier. 112" is the nominal value, but then there's reality. The good news is you can always change the location of the locating pin hole in the axle spring pads for fine tuning. The ideal would be to have the full body mounted, wheel/tire combo representative of the final selection, and chassis with a typical load on it. When that's done you may find that the alignment of the wheel/tire looks better moved slightly one way or the other.