I have a 67 Fastback and ive been on the fence for years whether to mod it from stock suspension..but the biggest problem is options. The camaro camp seems to have a long list of suspension upgrades...as far as the Mustang goes Ive only seen 3 popular choices.
1. Mustang II (which means chopping up the front unibody and removing the factory shocktowers...but which then gives you elbow room to shoehorn in a Modular Ford motor

)
2. Use a Coil-over redesign using the existing shocktowers (example Total Control or Global West products)...can also swap the coils for Shockwaves for air adjustability
3. Use the same old technology but with tubulars A-arms
At the moment im leaning with a simple coil-over system and hope that actually makes it fun to drive again