Another thing to do is to have a complete, terminated 68 pin chain with the motherboard as a 50 pin terminated end using an adapter. I have a few Quadras which are configured this way using 68 pin U2W SCSI to IDE adapters (80 MB/sec). The adapters wouldn't negotiate to single ended, narrow unless the bus was terminated wide. You can see my adapters and 68 pin cable and terminators in my Quadra 610 here:

http://www.macbsd.org/~john/quadra610/