The upper shock mount on NAs look like a solid piece, but they're actually two pieces of steel joined together by rubber. Over time, the rubber can fail leading to a collapsed mount. This means a lower ride height on that corner and a harsher ride. If your Miata is sitting a bit crooked or seems to be low, this may be a problem. Given the age of NAs, it's a good idea to replace these while doing a suspension swap.
You can inspect the condition of your mounts while they're on the car. Look at the top of the mount (the portion visible under the hood or in the trunk). There should be no gap between the center section and the rest. See the picture for an example of a collapsed mount.
These are the "long stud" style used from 1994-97, so there's enough thread to run shock tower braces and master cylinder braces. Fits all NA chassis from 1990-97. Sold individually, there's one mount per corner. Aftermarket part.