The problem with 73 mm BB's is you can't use the "jam" nut to secure the main nut for the installation. Probably a stabilizer bar will solve that problem. The other consideration is that some chainstays are configured around the BB in a manner which keeps the motor from "nesting" properly on the drive side. Additionally, if the frame doesn't have a "normally" triangulated intersection of the down tube and BB, ground clearance is impaired. I've only converted steel and aluminum "normal" frames and everything was easy.