Alex,
From the Software Download Technical Support page, you are looking at the correct CatOS image.
cat6000-sup2.6-3-5.bin
Catalyst 6000 Supervisor 2 Flash Code
cat6000-sup2k8.7-6-5.bin
Catalyst 6000 Supervisor 2 Flash Code
I guess the filename shouldn't matter. You probably have done this but I would suggest to check the features and changes (in commands, for example) between 6.3 and 7.5. This might have an effect on your current setup/applications. I have seen quite a number of new commands added in version 7.x. And your first reference is the Release Notes, of course :-)
Goodluck!