It was a Toyota 2000 GT. I heard a story that Sean could not fit into the coupe so they made a one-off convertible for the movie. Not sure if it is true.
The reason for cutting the top off the Toyota 2000 GT was not because he was too big to fit in, there were many Brit cars of that era that he would not have fit in. It was because they had a hard time to film him in the car. The car was taken back to Toyota where it was transformed into a convertible in 2 weeks.