I have a 92 ford ranger a we replaced heads,gaskets,thermostat and still gets hot and it blows a spark plug wire off

Replace the radiator--phosphate scale is likely blocking passages. Refill with a 50/50 mixture of antifreeze and distilled water. Read you Owner's Manual on how to properly fill the radiator and purge air from the system.

Extreme heat = high resistance. The resistance in the head may be so great that the electricity in the spark plug wire is trying to find a shorter path to ground. In the process, it rips the wire from the plug.