Replacing speedometer and odometer

I would check with your states DMV office to see what the legal requirements are for resetting an odometer which is normally a crime.

They can be opened up and wheels set to whatever number you want, or you can mechanically spin the odometer cable until it hits 100K, which will be the same as showing 0 miles. Couple caveats: your state will reflect "odometer discrepancy" or "exceeds mechanical limits" on the title, should the title ever be transferred in the future. Not a big deal as it happens all the time with older cars that have been restored, restomodded, etc. (This depends to some degree on your particular state, some are more strict than others.) "Odometer discrepancy" is noted on a title anytime the claimed mileage shown on the odometer is less than the last stated mileage at the time of transfer or issue. If you don't want to open up odometer you can always buy a reproduction one.

Hot to reset a mech. odometer on a 67-68 Mustang; this video shows the basic means, it will be the same on any early 70s Ford: https://www.youtube.com/watch?v=87kZQkKGSw8 BTW, this is only illegal if used to defraud a buyer or alter mileage shown on a title, it's also illegal to give false information to a title bureau. I've rebuilt a few older cars, which included a completely new dash and instrument clusters (or the old ones completely rebuilt), which resulted in an odometer showing 0 miles, but always stated correct mileage on title.