I own a 70 Cougar with an original FMX+351C and have changed transmissions over and over again. After doing it all I think the stock FMX is great as-is. Put some 3.50 gears in and you'll find the right balance.
I changed to an AOD and C6 then to a low-ratio C6 and found very little change in performance. And by the way my Cougar runs 11.20's in street trim so it's not lacking in power.
The AOD can be a lot of fun once it's set up but a stroker engine can cure a bad gear ratio any day!