It's not likely to be financially viable to convert an Ultra320 SCSI drive to USB. I wasn't able to find any Ultra320-to-USB adapters on the open market. Failing that, the least expensive Ultra320 male-to-male cable I could find was $45. To put that into context, a brand new Seagate 1TB external USB drive costs $50 at Amazon. So you would be better off just buying a new drive rather than finding an adapter for a used drive.
The new drive will be faster anyhow due to its higher density platters.