I'm pretty sure those locations were 1/4-20 for a while and then changed to Metric in the later years, likely 2003-2006. I am not 100% certain on that but for a 1998 I would say 1/4-20. Get phillips head screws so they will actually fit in the snubber hole.
yes, tire needs to be snug against them. This is a problem with the Morryde because using the regular TJ tire carrier at stock height will have a large tire hit the bumper. If you use the brackets to raise the tire carrier, it raises the carrier up and out which creates the too-far-from-snubber issue.
Check out
@pc1p thread on the issue and how he and others have fixed it with some welding work to the stock carrier. Might be a better option for you if you are able to get the fix accomplished.
Anyone notch an Exogate?