there are some 68 to 50 pin adapters that terminate the upper 8 data lines correctly, other than that, have the 68pin drives on the ends of the bus with active termination enabled.
the other option is to only have 68/80pin devices with a 68wire ribbon cable hooking them all together. then at least the upper 8 bits are terminated, and the host adapter with a 68-50pin plug is on the lower 8 bits.
