Had a simular situation with a long dormant 216 approx 3 yrs ago.
After trying every trick I could to get it started a compression test told me I didn't have enough compression in 3 cyl's for combustion. After pulling the head I discovered all the exhaust vales were burnt and wore and the intakes not in much better shape either. A head rebuild later it's been running strong since.
Make sure the valves are fully closing by ensuring they are adjusted properly (just set them cold, loose is good in this case) and if that don't do it do a compression test.
Good luck
Dave
