Here is a trick we use on positioning sheet metal on a frame we built from scratch. Cut a disc from heavy cardboard with the same size diameter as the tire you will be running---usually 25-29 inches and nail bottom of disc to a wood block. Set the cab-fender unit on the frame and and slide the very easy to move around dummy tire till you like the way it looks. Now mark the frame rail with a mark that lines up with the center hole in the cardboard disc and make another mark in the exact same place on the opposite frame rail. These marks will be your spindle center line no matter what front end you use, IFS or straight axle. You then have a point(s) to pull all your other measurements from. If going with a factory bed you can find the stock wheelbase, measure that distance from the front spindle center line and that will be the center line of the rear axle. When actually positioning the front and rear ends it's handy to have a rod or light angle with legs down to the frame and far enough out on each side to where a plumb bob can hang down for spindle or axle center line alignment. On some vehicles, like AD Chevy's the rear wheels were NOT centered in the opening and this is the time to correct it.

Evan