That 1/2" of rotation translates to about 8 degrees of timing. If you wound up on the 4th or 5th mark (each mark is 2 deg), that confirms 8 - 10 degrees of looseness in your timing chain. If you're certain on the stabbing of the distributor, then the engine may have jumped time. Even w/o jumping a tooth, a timing chain that loose could have caused all your original symptoms.