Check that your 80-to-50 pin adapter has a termination setting. Not all of these adaptors do (as I have found out) and it is a nuisance in a machine like a Quadra 700 where you probably have no other SCSI devices.

My guess is that you are correct. However, I check for 8-bit SCSI support in the docs otherwise you are going to need a FWB JackHammer or an ATTO SiliconExpress IV.