The 67/68 Mustangs are considered the best years for building as they had a better frame/suspension than the previous years. (built on a Ford Falcon frame/suspension) In 69 Ford shrunk the engine compartment so that only small blocks (up to the 351) would fit. The 67/68 could hold a 427, 428, 429 block.
The fastback is the most sought after model as it is stronger and more aerodynamic than the coupe and the convert. If you can find one, it will be someone's project that they lost interest in. Any you find in a junk yard will be body damaged with probably frame damage and more than likely lots of rust out.
If you want to really make a sharp 67, learn how to make a fiberglass mold, find someone with a good body who'd like to go into business with you and copy the body. You can then sell body kits to other restorers, or build a tube frame and sell entire car kits. (There is someone doing that with early Vette reproduction kits and another doing 55 T-Birds.)
The 67/68 were close in size to the LTD II's from the mid 80s. (lots of them in junk yards for cheaper parts.)The running gear/steering, etc. from an LTD II is cheaper to buy and actually handles better than the original Mustangs. You could use the LTD II frame, but a tube frame would be stronger and lighter.
Hmm, I may have just decided on a new business for myself!!