The biggest engine Ma Mopar ever put in the Formula S was the 383. In the 69 Cuda line they also installed the 440. As far as the 67 that you're looking at buying, the most sought after parts are the grilles, fenders, taillights, and trunk trim. It would have a lot going for it if those are in excellent shape. Maybe $2000-$2500 without seeing it it's hard to tell. Maybe more if it has an 8 3/4, 727, uncut dash bezel, headrests(rare), am/fm, woodgrain wheel, etc. The notchbacks are really cool looking at not as common as the fastback. It definitely sounds like a keeper if it's a running 1 owner. Good luck. Jim