Hello fsangre.
As they say, "education costs money, ..."
It looks that you've scored relatively rare Ultra Wide SCSI version of Plextor.
The good news are that it could be adapted to 50 pin cabling, since Narrow to (Ultra) Wide adapters exist, but the bad news are that UW device needs to be the last one on the Narrow SCSI chain with its termination activated. If I remember correctly, in 8500 the hard drive is last on the chain.
If you can change your cabling this way:
motherboard -> hard drive (termination OFF) -> Plextor (temination ON)
the you'll be fine.
Just pay attention to 50 pin to 68pin adapter form factor, so that it does fit and not interfere with 8500s power supply and other cabling.
Google for "scsi 68 to 50 adapter". From my experience, DataMate adapters are the most compact. Check this ebay listing: 322442924890
As to the audio connector, yes they are different, but the odd one here is Apple/Matsushita, not the other way around.
Besides, you've said that you need your Mac with working CD-ROM for some super-duper scanner (drum?) and not for playing audio. Wasn't it so?