Will 7 1/2 X 15 fuchs fit under a '69 912 OK?

 

I've never heard of a 7.5" wide Fuchs (not to say there isn't such a thing). You might be able to fit 7x15's in the rear, with 205 or 215 tires. In the front, you're generally limited to 6" widths on the early narrow body cars.

I think 15x6 Fuchs with 205/60's or 16x6 Fuchs with 205/50's or /55's is a natural and well-balanced setup for the early lightweight cars (at least until you get up over the 200hp mark and need more rubber on the ground.)