65-66 pedals are all the same, the leverage point does not change on the pedal itself for those years...however that does not tell the full story.
A factory single bowl "fruit jar" manual brake master cylinder has a 1" bore.
According to the Leeds website so does their kit:
https://leedbrakes.com/i-20507149-hydraulic-kit-power-brakes-64-5-66-mustang-auto-trans.html
A factory dual bowl manual brakes master cylinder has a 7/8" bore.
Changing the bore size will change the pedal pressure AND the throw. What this means, is that if you have manual brakes and swap from a 7/8" master to a 1" master cylinder while NOT upgrading to power brakes, you will feel DECREASED pedal pressure and a longer throw, . If you add power brakes to the mix, the pedal feel will be even worse.
In your case, you are simply adding power brakes...assuming you started with a 1" single bowl master, your brakes will now feel softer but the throw will remain the same. If you started with a dual bowl master, you will feel decreased pressure and a longer throw.
There are other things to consider though...the master cylinder is only ONE end of the system...the other end is the brake caliper piston size...the larger the caliper piston, the more fluid is needed move it...a larger piston reduces pressure and the brakes will feel softer.
Do you see the problem here? Just because you throw some components together and offer it as a "kit" doesn't mean they are optimized. Do a google search, you will find page after page after page of vintage mustang enthusiasts unhappy with the result of power brake kits....this is because components such as master cylinders and wheel cylinders(or caliper pistons) are so often mismatched.
I hate to diagnose by the internet, you make no mention of what the pedal actually feels like....but if the feel is good and the travel is just too long, the solution would be to increase the size of the piston on either the master cylinder side or the wheel side....but if the brakes are feeling soft and its assumed they are correctly bled, you could reduce the master cylinder size to increase pressure.
In the end...I never recommend swapping to power brakes unless you do the homework and put together the parts yourself...a lighter pedal is just not worth the effort to me.