The majority of A300B4 frieght aircraft in recent or current use are either BAe or DASA conversions from pax aircraft. I believe the first conversion was at ASL Lemverder and possibly a few at Sogerma.
The BAe Filton conversion was an FSI STC, the method of floor strenthening was reinforcing the original floor beams, the DASA conversion at Dresden removed the floor beams and replaced with brand new A300-600 units.
The most obvious differences were one used an electric motor to open the main deck door and the other used the hydraulics, however from an engineering point of view, the more expensive DASA conversion was by far the best, the FSI conversion required further modifications to prevent structure cracking and the flooring gave problems in service.
The weakness on the aircraft was the main frame 40 and 47 to wing attachments, with cracking in the fastener holes, this was a very expensive fix.
The in service problems came from replacing the steel brake units for the much lighter carbon brakes, this caused over heating of the skydrol hydraulic brake fluid causing the fluid to become a very acidic jelly, in turn causing internal corrosion problems within the hydraulic components.
Other than that the aircraft was a very reliable freighter if operated and maintained with abit of TLC.
Best regards,
om15