The 68B Cabriolet has a straight windshield like the Coupe, but there may be some differences in the lower sub-rails where the cowl connects to the wood body sub-rails in the Cabriolet. For example, the metal cowl sub-rails of the Cabriolet attach to the wood body sub-rails with wood screws. In contrast, the Coupe has metal body sub-rails, so the cowl sub-rails attach with rivets.
My guess is that you can make the 68B Cabriolet's cowl fit the Coupe. However,
the 68C Cabriolet has a 10-degree slanted windshield and its cowl will not work on the Coupe. No production Coupes have a slanted windshield.
If you opt to attempt grafting the 68B Cabriolet's cowl to the Coupe, tack weld the Coupe's doors in place while making the graft to assure there will be excellent alignment of the doors to the body and cowl.
Good luck with this project.