Firstly, 8 hours is extremely optimistic for the drive from Munich to Split. I have driven a similar routing (Bregenz, Austria to Split) and I would say that Munich to Split is a minimum of 10 hours + breaks.

I don't know about Grossglockner, I'm afraid. Although I live in Austria, I have never actually been there.

As for interesting things on the way, I would recommend either Lake Bled or Lake Bohinj. Both are a short distance north of Ljubljana. Which one is better is a matter of personal preference. Bled is more touristy and developed, whereas Bohinj is more rustic and natural.