Hardware mods: kludging a conversion from U320 SCSI to SAS/SATA
Solution 1:
Realistically speaking, you're doing your employer a disservice trying to drag a machine with such old technology into the future. It's time to buy a new machine. By the time you spend the time cobbling together this bespoke solution you'll have spent more of their money than just buying a new machine. You're also leaving a major headache for the next person who has to support it.
(I say this as a really, really cheap guy who is fully on board with the idea of using out-of-warranty or off-lease equipment in production roles, provided that adequate spare parts are available on-site.)
If you really want to continue using this machine buy an external disk storage unit of some type (DASD, iSCSI, fibre channel, etc) and attach it to the machine. At least then you're not leaving a horrible nightmare for somebody else to support when you're gone. It can also be attached to a new machine in the future, allowing it to outlive a host server that's already likely well past its prime.
Doing anything inside the box isn't financially realistic and creates a nightmare for the future. Don't do it!
Solution 2:
This has definitely been. covered. here. before.
The short answer is that it does not make sense to pursue this in any way today. It's irresponsible to consider keeping a 10+ year-old server on life-support in this manner.
The historical context is that the industry moved away from Parallel SCSI (U320) to SATA and SAS beginning in 2004. New servers designed in and after that time defaulted to SAS/SATA backplanes. PCI-Express was becoming the standard as well to replace PCI-X. This also likely means you're on the wrong side of the 32-bit/64-bit fence. There may be other compelling reasons to upgrade hardware.
Remember, there are practical limits to work with here. You're at the edge of what your original hardware is capable of. The cleanest approach to handling failed disks on the extant server is to find like-model U320 SCSI equipment to replace the drives as they fail. Ebay is the best resource for that. There's less risk and cost involved, and it leaves you with time to focus on better long-term solutions, like virtualizing or migrating this system to modern hardware.