The front frame rails on an F body are not parallel with each other. At the point the 4 front crossmember bolts attach onto the frame rails, the front 2 bolt up mounts are 4" wider, center to center, then the rear 2 bolt up mounts are. I believe the outside dimensions at the front are 38" and at the rear bolt mount the frame is 34" outside. The frame rails are also not flat nor are they square or rectangular shape. The rails are also not uniform in size or shape from the radiator to the transmission crossmember. If your planning on hanging anything other then the factory crossmember on the rails, expect to replace the rails with tubing. If you do that, do yourself a favor and replace the transmission crossmember at the same time.

I'm going to guess the hub to hub width is close to 64" wide, it is about 4" wider then the hub to hub on the front of a Dakota frame. I junked the last F body crossmember here a week or 2 ago, so I can't measure for sure. Gene