1966 mustangs did not come with V-6s they came with straight 6s and 289 V-8s Value is decided by options ,condition and model ( GT , Convertable, fastback 2+2 etc...) all originals and unmolested cars are of increasing value, restomods are also gaining in popularity and value . Check with your local mustang club or the MCA (mustang club of america ) and they will be able to answere your questions .