With stock rear flares you won't be able to fit more than a 7 inch wheel. In the front a 7 inch SHOULD fit without rubbing, depending on your ride height.
No offense intended but I think Boxter wheels would completely ruin the classic look of the early 911. Obviously, it's your car and you should do what makes YOU happy not some clown on a car forum.
AS well, the spacers you will need to run to correct the late moel offset will add weight at the four corners which MAY cause some poor handling traits.