The 763 on transatlantic flights has the fully flat sleeper seats in F, similar to the 777 Pacific version (not the Flagship suites). The C class seats are ok too, but not fully flat. If you have booked an award ticket in C, you cannot just "upgrade" to F. You will have to change the award booking to an F class award ticket if you want to ride in F or pay the difference between full "J" and full "F".
Regards.