Having done a bit of surfing I reckon the gt screen is about right - the angle looks the same and the B GT and SWB are the same height.
Major differences are that the Ferrari door goes much further forward under the scuttle than the MG door does, and the angle of the door glass isn't quite the same. I think it could work, especially for those of us that don't really like modern donors...
Oh, nearly forgot, anyone know if the B GT roof is structural? I'm thinking cut it short to match the length of the Ferrari roof and graft on a fibreglass panel to give the right angle at the back end.