Any of the G3/G4 PowerMac towers made after 1998 will work well as a bridge machine. You won't have 400K/800K floppy access, but, as long as you have a SCSI card installed (I personally use an Adaptec 29160N in whichever G3 or G4 I have at the moment), you can either stick a 50-pin ribbon cable and a hard drive power cable out an empty slot opening in the back of the computer, then run a SCSI drive hanging off the back, or use a 68 pin to centronics SCSI cable with an external hard drive case. Copy your software from the MDD to the hard drive, then reinstall said drive in the 68K Mac you need to get the software files onto.
Alternatively, you can use either a zip drive or a jaz drive to transfer files to and from the MDD and your older Macs.