I've never seen a complete step by step set of instructions for replacing the whole front clip but there is a dimension drawing in the 1968 Ford Service Manual for what it is supposed to be when done. One of my 66 Mustangs needed the front frame rails straightened and the 66 book was what the frame shop used. In California, a certified welder is required to do that type of work as it gets very extensive. Have you ever heard of or seen a car 'crab' down the street? Any front clip from a 68 Mustang will work but DO NOT USE A 67 FRONT CLIP as the engine mounting holes and engine mounts are different. Be very careful when you cut off the old front clip as there are hidden VIN numbers on the fender aprons covered by the fenders and by some careful body bork, they can be retained.