How is the fan belt?
This is an oppty to learn how to change that, as well.
I think you need to remove it anyway to get the A/C belt off.
For fitment, I think a belt from the dealer is the way to go.
And order a few washer spacers, in case you're missing any.
That means an oppty to learn how to use the parts catalog for your year.
See illustration 105-00, part #12 and #7
http://www.porsche.com/all/media/pdf/originalparts/usa/911_USA_74_77_KATALOG.pdf