I converted the backup lights on my ‘64 to stop lights using the later red lenses. It worked OK, certainly better than simply relying on the stock tail lights, but frankly not nearly as ‘seeable’ as a third brake light…. maybe that’s because the third brake light is higher, and more in line with the vision of the driver in the following car.
PS… my ‘85 Avanti has the back up’s changed to stop lights (done at the Avanti factory).. really not as effective as a third stop light in rear window!