I am still fighting this same battle with my 216, and haven't won yet. This is what I have done and should establish a base line for your diagnosis.
Start with a compression check and eliminate low compression and bad valves.
Replace the Points, plugs, condenser, rotor, cap and plug wires, to eliminate any of those issues.
Set your timing IAW the specs.
Make sure your firing order is corrrect, 1-5-3-6-2-4.
Check your vacuum at the manifold. Should be around 18.
Check your vacuum Advance. Apply vacuum to the line and it should hold steady.
