68 spindles on a 66 mustang?

What are you trying to do, and what is your real question? What brakes are you trying to support?

Are you trying to run and after market brake system, or an oem type brake system? After market disc brake systems such as Wilwood add appx 5/8 inch to each side requiring a different backspace. Baer disc brake systems don't alter the width.

You can put 68 spindles on your 66, you'll need 68 outer tie rods. You can put 70 spindles on your 66, you'll need 70 outer tie rods.

Opinion only!!! Most people doing mustang suspension/brake work would know a 68 spindle fits on a 66 mustang by just changing the tierod. If they can't be trusted to know that, I'd be just a little afraid of their product.