I want to do this car justice...Does anyone have any idea if a Mustang Coupe, built on April 17th, 1964, with a V8 4V ("D" code) is better restored to factory specs or if building a Honda killer out of it would ruin it?
To me it would depend on how complete and original the car already is. If it's already been modified, I'd continue down that route (I like modified cars better *LOL*). If it's pretty original and pretty complete, I'd probably vote for the concourse route.
In either event, though, I wouldn't do any mods to the car that can't be "easily undone".
Ford destroyed the 64.5-66 production records years ago. It is impossible to know exactly when your car was built. All you can do is rule out dates by inspecting components on the car such as sheet metal date stamps, drivetrain date stamps, etc... If you find a date on an original component after 4/17/64 on the car, rest assured that it couldn't have been built that day.
Unfortunately, this is the situation we are in without the production data.