It could easily be a totally different thing that caused her to quit then and to be stuck now. It could have jumped time or the distributor advanced and got stuck. It is common for engines to get stuck from sitting. I would try something like Marvel Mystery Oil. If you can't turn it with prying on the ring gear, I would suggest pulling a head, getting a 3.98 seal driver and pushing down on a piston with something like an OTC 938 Push/Puller and use head bolts for legs for the push. I do this on stuck 302 engines with 4" bore. The 305 has a 4.25 bore, but I think you wouldn't hurt anything with a 1/4 under bushing driver. Pulling the heads is not a major undertaking, and if you can get her to turn 360, you might not have to do much at all to get her running.
Who knows what happened to it in the first place. I remember going out on a call to a person who had been driving all day with a dead generator. Finally the motor quit and of course the it wouldn't turn over. A boost and new generator brushes got him going. Had he parked her then. The motor would probably be stuck by now
