@HammerD
I have the
AEC-7720UW (Ultra-Wide SCSI to IDE Bridge) in my A4000 (on the CSMK3 UWSCSI).
For a standard A4000T SCSI you could use the
AEC-7720U (Ultra SCSI to IDE Bridge). Beware though that it's designed for CD-ROM (5.25") drives, so it sticks out a little from 3.5" drives. (see picture for more information)
You could probably also use one of the "Smart Bridge" models, but I haven't used them personally so I can't vouch for them.
As for speeds, I don't know what kind of speeds you would expect. Like I said, I get consistant 10MB/s from a Western Digital 120GB (WD1200JB) on the CSMK3. Standard SCSI will be much slower, but you'll get the advantages of DMA and cheaper HDs.