GB, in BSA Twin Restoration book it specifies for:-
356 camshaft: Inlet open 42, inlet closed 62, exhaust open 67, exhaust closed 37.
334 camshaft: Inlet open 30, inlet closed 70, exhaust open 65, exhaust closed 25.
Are you timing it as before when you had the 334 fitted? Look at timing it as a 356.