1981 FORD F150 Tires Problems

I PURCHASED FOUR USED SX 5000 STARFIRE (COOPER) 255/70-15 TIRES FOR MY 1981 FORD F-150 IN MARCH OF 2010. THE TIRES WERE MOUNTED TO WHAT I BELIEVE ARE 15X8 STEEL WHEELS BY THE INDEPENDENT TIRE SHOP THAT I PURCHASED THE TIRES FROM. I THEN DROVE FROM SAINT PETERSBURG, FL TO AUSTIN, TX ON THE TIRES WITH NO PROBLEMS. OVER THE NEXT TWO OR THREE MONTHS, I DROVE ABOUT 500 MILES ON THE TIRES WITH NO ISSUES, UNTIL ONE DAY ONE OF THE FRONT TIRES WAS FLAT IN MY DRIVEWAY, WITH THE TREAD SEPARATED IN ONE AREA. I REPLACED BOTH FRONT WHEELS WITH SPARE WHEELS AND TIRES (UNKNOWN EXACT SPECIFICATIONS, BUT SMALLER), AND WITHIN A COUPLE OF WEEKS, ONE OF THE REAR TIRES (AGAIN, IN MY DRIVEWAY) WAS FLAT, WITH THE TREAD SEPARATED. I PLACED TWO OTHER SPARE WHEELS/TIRES ON THE REAR AND PLACED THE SX 5000S IN MY GARAGE. WITHIN...