Why won't my 1990 F-150 start?

I had replaced the positive-to-solenoid and negative-to-ground battery cables and my 1990 F-150 4.9L started great. BUT after I replaced the distributor cap the next day it has labored cranking and won't start. I checked #1 spark plug against the valve cover and can see it spark. There is a fuel smell and I can hear the fuel pump running. Replaced the rotor: still won't start. Replaced the spark plugs: still won't start. Replaced the plug wires: still won't start. Replaced the battery (under warranty) (had a low cell in a battery once on another car, this was the diagnosis, and a new battery cured it): still won't start. Replaced the starter solenoid (accidentally broke it): still won't start. The new battery had all the cells ~1/8" low so added water and placed it on the charger: still won't start and still has labored cranking. What could it be?: bad new battery? starter motor? solenoid-to-starter cable? ignition module? ignition coil?