I used a B&M Starshifter in my last Cougar. 351C with an FMX. Never had a problem with it and it looked good too. Confused the heck ou of people when they saw the "4 speed shifter" with no clutch pedal! : ( http://www.bmracing.com/noflash/index.html )
I didn't have to make any changes to the shifter, but I had to rework the shift arm o the tranny. The shifter came with new bolt on arms for the C4 & C6, but the FMX was pressed on to the shaft. Not a big deal. Once I had it installed it worked flawlessly for years.