If I understand what you last said:
[...] would it be better to use GParted, shrink my existing partition to 40 GB, and create another Macrium image that can be restored to a larger or smaller partition no problem? [...]
you are coming around to what I first suggested:
A safer, more flexible way might be to shrink your partition to the minimum size that will hold the data in it before making your Macrium image. That way you can more easily restore it to any partition that will hold the data.
I really think this is indeed your best, safest, most flexible and reliable option. I also agree that it will be best to do a good, thorough, defrag and system cleanup beforehand, including moving things off the system disk that are better kept on a separate data disk, deleting temporary files that are no longer needed, etc. There are many free partitioning programs out there, including GParted. I'd be glad to tell you which one I've used in the past when I have had to shrink a system partition, but I no longer remember. It can
be done successfully with no data loss. Once shrunk, Macrium Reflect should be able to do a good job for you. I have used it successfully in the past myself. One thing to be aware of, if you are not already, is that when Macrium Reflect clones a partition/disk, it really duplicates it. In other words, you can't have both the original and the restored clone both in your system at the same time, because they both will have the same "Volume ID"(??) and Windows will not be able to tell the difference between them. Sorry I'm not sure about the exact terminology, my memory is not what it used to be, but I had this problem a few years back when I tried to reuse the original disk, for other purposes, in the same system I had installed the clone. I found a utility that was able to reset the "Volume ID"(??) so all ended up good in the end.
Cheers and Regards