1990 CHEVROLET C1500 Structure Problems

1990 CHEVY C1500 1/2 TON STEPSIDE PICKUP TRUCK. GENERAL GRABBER AP TIRES. SIZE 235/75/R15 M+S MADE IN MEXICO OTHER NUMBERS ON TIRE:OK-47410 P5501 25855 021943. 147,300 MILES ON VEHICLE WHEN LEFT REAR TIRE BLEW OUT AT APPROXIMATELY 70MPH. TREAD SEPARATION CAUSED SIGNIFICANT DAMAGE TO TRUCK, RIPPING FENDER MOUNTS OFF IN 4 PLACES, TEARING THE FUEL FILLER NECK FROM FUEL DOOR OPENING AND TEARING OFF THE GAS CAP. FIBERGLASS ON BOTTOM OF STEPSIDE FENDER WAS HEAVILY DAMAGED. TRUCK WAS IN OTHERWISE GOOD OPERATING CONDITION. TRUCK HAD LESS THAN 100 POUNDS OF CARGO; TIRE PRESSURES WERE VERIFIED TO BE 35PSI AT MOST 3 WEEKS EARLIER WITH NO LEAKS. PRESSURE GAUGE ACCURACY WAS VERIFIED BY COMPARING TO SEVERAL OTHER GAUGES. TIRES WERE INSTALLED AT 127,445 MILES WHEN DEFECTIVE FIRESTONE TIRES WERE REPLACED. TIRES WERE ROTATED AND BALANCED AT 144,000 MILES. SPARE TIRE WAS INSTALLED AT ROADSIDE. BLOWN OUT TIRE IS STILL IN TRUCK...