NEED HELP WITH SHENMUE

I just downloaded SHENMUE disc 1 for the Dreamcast. I read the kal-smu.nfo file, it says to use an 80 min. disc. An 80 min disc is a 700MB disc but the .cdi file is 759mb I cann't ues ISO BUSTER ON a cdi file. How do I back up this game on an 80 min. disc that holds only 700mb? I read the newbie guide but the guide had no info about this. Can some one PLEASE help me! This is one game that I must have!