Why 351 windsor motor turns over but won't start and no spark?

If there is no spark on any plug, your problem is most likely in the distributor or the coil. I'd start by replacing the cap and rotor. If you already tried that, try again. Sometimes the parts guy can transpose a number and you get a rotor that is just a hair too short. (happened to me on a Ford 302 HO). If that doesn't work, try replacing the coil and wires. Then on to the points. And that's all that makes up your ignition that would affect all 8 plugs at the same time.