I stumbled upon the information while researching for a friend that was in the same situation you are. He has added a lot of HP to his convertible and felt the shoulder belts could be very beneficial at this point.
Also, I have yet to see a convertible with the factory installed shoulder belts in any convertible I have encountered.
1967 RA convertible 1968 H.O. coupe - sold 1976 Trans Am - sold 1985 Trans Am - sold