What type of hp are we talking here? Unless you are planning on 500hp or more the FMX is just fine. If you are planning on more than 500hp a C-6 will need some work done to it as well. Both transmissions can handle quite a bit of punishment. The FMX is cast iron so it is heavier than a C-6, but I believe that the FMX robs less hp than a C-6...someone correct me if I'm wrong.
I've known a few racers to run a C4 behind a 351. I have I know a stock one is good for at least 450HP, if built right. Likely more if some changes are made. I agree about the parasitic losses. Blueprinting and a few parts changes can make a big difference in those numbers as well.