Technically the C8 decal is considered correct, however, I have seen multiple 1968 GT cars (unrestored/original) with the C7 decal. If you do not have your original trunk decal to document, I would go with the C8 decal. If you have documentation of your original, I would replace it with the duplicate of the original.
MCA rules do allow for the painted standard steel wheel as a spare, however, I believe the spare wheel should also be a styled steel wheel for a number of reasons.
Danny