FORD found on road dead!

I would check a voltage drop through the positive and negative cables. Start by hooking the positive side of a digital volt ohm meter to the positive side of the battery. Hook the negative side of meter to negative battery terminal, attempt to crank engine and note voltage. Now take the negative side of the meter and hook to a known good ground on the engine near the neg. cable. again attempt to crank engine. This voltage should not vary by more than a couple tenths of a volt. If the voltages are comparable cable is ok if not then negative cable is bad, next do the same procedure with the positive cable, first at the battery then at the starter post again compare voltages, If voltage is substantially lower at the starter end of the cable then the positive cable is defective. if both cables show ok and you are getting full voltage down to the starter (at least 10 volts minimum) then I would say that the starter assy is your culprit, if at any point you lose voltage track backward to your battery and where you loose your voltage this is your bad connection, good luck.

As above, cables were checked, voltage included , & cleaned. The crank was hard to turn even with the plugs out. I went to purchase new starter (solonoid included) but the mechanic tested it & it was fine.

Sorry did not see where you had done a voltage drop test on the cables. anyways did not see but did you attempt to turn vehicle over when the belts were off? If you take the belts off and the engine turns over fine it is possible that you could have a resistance in the belt area (water pump bearing, power steering pump, etc.) if you remove the belts and the engine still spins over extremely dificult with the plugs out (which it does infact sound like you did) it sounds as if there is internal engine damage, but remember there is going to still be quite a bit of resistance even with everything unhooked, before calling this the culprit I would do a amperage draw test on the starter itself, even though the starter shows good off the vehicle it may be drawing excessive amperage when a load is put against it, with everything hooked up and ready to start you should not see more than 250 to 300 amp draw from the starter, if there is more than this you should replace the starter. All of these tests should be performed at a very reasonable rate by a qualified technician, I use a VAT 40 which tests the entire system by hooking amp lead and voltage leads to the car and cranking it, any reliable shop should do the same, and we usually do this as a free service due to the fact that it only takes about 5 minutes to do the entire test. Other than this I cannot think of anything to tell you that you have not already covered yourself. Sorry if this does not help but good luck and thanks.