i purchased a modifed veihicle from ford with extended warranty from an outside plan not ford. the cars motor blew up and they refuse to cover it

I am fairly confident that your did not buy a hopped up Ford car from Ford directly, as opposed to "at a Ford dealer", as manufacturers dont sell non production cars that blow up the engine to do over pressure, excessive torque on the tranmission/transaxle, etc. and don't provide third party warranties for such things. If you  It is a well known that if you mod an engine or performance outside of factory spec, whether it being a chip/computer replacement, altering air/fuel ratio, running NOS or race fuel,etc, its considered abuse of the engine and voids factory warranty. Absent an actual manufacturer warranty, that covers the mods or they are factory mods, you are likely out of luck