I have a '68 convertible that is missing the shakers. I've added subframe connectors and rebuilt the suspension (plus front and back sway bars). I don't seem to have any vibration problems at speed, 80mph or so. I agree with Earlybird, however, that the engineers didn't add the shakers without a good reason. I'd like to try them someday and see (when $ are not a problem...)
Waste of money (and weight) in your case. The purpose is to dampen out vibration due to the lack of structure without the roof. The subframe connectors stiffen the structure. Which is why you don't notice a vibration issue.
