As I said the odds are that it will be OK, just be aware that the weather in the valleys is no guide to what is happening higher up. I have been over the top when it was 25 c lower down and just 1 c at the top. Check the weather on the road itself. The webcam at the top is one of the best ways of checking. If you use the Grossglockner website you will find more up to date info on the German version. They often either don't mention road closures on the English version or put them up hours later than on the German pages. This also applies to special events, cycle races, tractor rallies etc, which are often ignored on the English version but listed on the German pages. Usually these only involve short term closure of sections of the road but forewarned is forearmed so you can plan to avoid (or spectate)
From Bled you can go over the Wurzen pass (pausing to take the obligatory photo astride the turret of the Russian tank!) skirt around Villach, pick up the B100 but turn off at the second roundabout, over the river Drau and on to the L37 and then L39 to join the B98 along the north shore of the Millstattersee. The floating bar in Millstatt is a great place to stop for a coffee. Then re join the B100 ( via B99 and the L9) before picking up the B106 all the way to Winklem and the B107,the start of the Grossglockner. Even in august you will see almost no traffic on those roads. The B106 and the first sections of the B107 are a fast roads with open sweeping bends but watch out for the police in the villages. A favourite spot on the B107 is just after you pass a large waterfall on your left . They wait just beyond a petrol station on the right and catch over excited bikers who can sense they are nearly at the exciting bit and miss the limit signs.
If you have time its even better to turn right when you get to the B98 and then ride the Nockalmstrasse to join the B99 (go south to pick up the L9 and B100). This will add at least an hour to your journey but the Nockalmstrasse is one of the best roads you will ever ride.
John