The only on that works well on that car is an original Bendix type that would have come on 69 Mustangs.  It requires a power brake pedal (see my FAQ page on boosters and pedals).

I have them for $300 for the set.  MC is extra.

I also have some of those 9" boosters, but I rarely sell one unless someone is just so price conscious that good judgement is tossed to the wind.  That booster also requires a power brake pedal for it to work.

You will be much happier with the performance of the original style booster and correct pedal ratio for your car.  The aftermarket boosters like that 9" version are nowhere close in performance to the Bendix.
A 67-68 would have originally been fitted with a Midland-Ross booster if it came with disc brakes.  The Bendix is a later booster that bolts up to the same bolt pattern and uses the same power brake pedal as the Midland uses.  If you can find a Midland, that would also be an option, but in my opinion, the Bendix booster is the one to have.

Yes, it clears.  That is only an issue on 65/66 cars.