First off, check the points, condenser and timing if you haven't already. Also, distributor cap and rotor. Leads and plugs too but I guess you've done that. These are the most common causes of your type of problem.
If that and what they ^ said doesn't fix it, look at fuel issues (blocked filter, carb jets, pump)
A haynes manual would be handy if you don't have one already.
Not quite sure what you mean about the leak in the spark plug well, maybe worn valve guides/seals?