I have an Early 2008 Mac Pro and new 8TB drives like the one shown in the photo at the top of this page. The two mounting holes closest to the drive's SATA connector fit the sled just fine. The other two, not so much. What to do?
Many years ago, I discovered that the best way to fix something was not to try to duplicate the manufacturer's approach but to come up with a new one based on the tools and materials at hand. So, this is how I cheaply, easily, and securely mounted these drives:
With the sled detached from the drive, remove the two mounting screws and grommets that don't match the drive's mounting holes. The easiest way to do that is to push down on the threaded ends of the screws with the blade of a flat-head screwdriver. That opens two relatively-large grommet holes on the sled.
As a substitute for the thickness and cushioning of the grommets, stick a small piece of similarly-thick tape or, using two-faced tape, other resilient material beside each of the grommet holes, on the side of the holes farthest from the usable mounting screws.
Next, screw the sled to the drive with the two usable mounting screws, making a solid but somewhat floppy connection.
To eliminate the flop, go to your local grocery store and buy some lettuce or other greens that are bundled with a long (about 20"), sturdy, plastic-covered-wire twist tie (alternatively, but potentially not as strong, a single strand of Ethernet wire).
Shape the twist tie into a "U" approximately the width of the drive.
Hold the drive in one hand with the sled-side up and the open end of the sled facing away from you. With your other hand, thread one end of the twist tie beside the drive, into the adjacent grommet hole, and back down beside the drive. Do the same with the other end of the twist tie on the other side of the drive.
Shape the twist tie tight around the sides and bottom of the drive and pull it tight against the cushioning material on the sled. Tie the ends of the twist tie tightly together on the wide, bottom of the drive with a single, full twist. Wind the ends of the twist tie securely under the resulting loop. Trim off the excess.
Tying the twist tie makes a "knot" that can be too big to fit between the bottom of Drive 1 and the top of the fan housing below it. For Drive 1, place the "knot" on what will be the bottom-left corner of the drive when mounted in the computer, so the "knot" fits in the space created by the curved corner of the fan housing.
I tried tying a drive to a sled with other materials and configurations, but none of them worked. Particularly, anything placed on the inside edges of the sled "runners" blocks the sled from sliding onto its mounting bracket.
If necessary to reinstall the removed screws and grommets, wrangle the screws out of the grommets. Ease the grommets into the grommet holes, then push/thread the screws into the grommets.
Happy Early 2008 Mac Pro computing!