You don't have to be an aeronautical engineer to comprehend that a converted pax -400 will not carry the same load as the pure freighter. If it could then Boeing could shut down its -400F production line.
The converted airframe is heavier and more restrictive; quite obvious just by looking at the upper stretched deck, by looking at all the window plugs and by looking at all the deactivated pax doors. The typical payload "penalty" is from 8 to 15kgs [1000s], depending on specific modification certifications. Although, an exhorbitant fee can be paid to Boeing for "paper" recertification of higher payload cabability.
I'd suggest you might want to do a little research first. Using Boeing's figures the 744F has a max structural payload of 112.6 Tons, while the BCF is 113.5 Tons. The 744F has a MTOW of 875,000 LBS while the BCF has a MTOW of 870,000LBS. You might want to look at
http://www.boeing.com/commercial/747...00sf_prod.html to check your thoughts.