Who knows if it is a Gran Sport, it's hard to say. Is the battery mounted on the drivers side? Being it's a convertible it already has the boxed frame (all Gran Sports had a Convertible frame regardless of body).
If it was a 64 it would have a down flow radiator. There were no Gran Sports in 64, in fact they were introduced in Jan of 65 so they were a mid year introduction. The brakes are different the wire harness is different, there are a number of things. So far it looks like it is very possible, but simply has a 64 front end.