Had a similar situation shortly after aquiring my '71 280s, "ran fine day before, now it won't even turn."

Ever heard of "vapor lock"? I was told then that this was my problem and to pull the spark plugs as vapor lock creates huge pressure in the chamber to the point of "locking" the piston. Though I didn't even understood what was told to me what I did was to manually turn the crank "backwards" a full turn or two. Once I did that it cranked and started just fine. Also it never happened again and that was about a year ago!

Don't know if that is your problem but maybe it's something to consider. Can you turn it manually either way?