I can't quite do the trigonometry to figure out your radius from the figures you gave, but someone here can. Anyway, it looks to be pretty small radius for any block. I'd form the curved portion with some bender board and wet drywall and pour it solid. Multiple layers of thin plywood or paneling will work too. Sometimes you can buy damaged sheets for real cheap or pull some out of a dumpster.
You'll need some of these and they will help your stone veneer as well.
They tie the inner and outer form together. Normally you just break them off after the forms are stripped, but as I said, they can have a secondary use.