Here is my '72 with Boxster wheels. 16x6 ET55, 16x7 ET40, 205/55, 225/50 Dunlop Star Spec, 1" bolt on adapter on the front, nothing in back. I use it for street and DE. I have had these wheels on there since around 2000. They work great, no rubbing ever.
Naturally, your '68 is the "super narrow body", so I can't say if these would fit as well on your car.
Good luck!
Roger