I prefer to upgrade to 7.6, as the policy exceptions make the conversion much easier. The caveat on that is that 7.6 considered controlled release. You can still get support for it, but there may be limits on that support that I have not found.
As far as IronMail version, there are no significant changes that would effect how the the upgrade happens since 6.7.1 was released. Support has some tools that can be used that require a more recent hotfix, and I have some that should work on anything 6.7.2 and newer (6.7.1 if you don't want your certificates).