Im doing the same with my cougar at the minute using the Coupe brakes before my turbo lump goes in.

I know of two experiences with large brake conversions on cougars as I run the uk owners club.

1st was on a supercharged v6 and that had 325mm Baer 4 pots. He reported a very poor feel to the brakes but no twitching.

2nd was a 3litre conversion and he had willwood 4 pots and reported they were very very stable under braking.