IF I had your car & was trying to keep it stock looking(as you have done)..I'd go back with either a R302,or Dart blocked 347/352/363. IMO,the way you modded the car was very tasteful & kept the original appearance
You can most likely bet on going to a 3-4" cowl hood,drop motor mounts,different intake,headers...ETC if you go 408. A 347/352/363 in any of those blocks(with your top-end parts) will haul a$$ in that lighter car,and be very,very durable..without changing intake/exhaust/hood...ETC!!