That serial number reveals that the engine was built at the Flint plant (F) on November 21 (1121). The J suffix is for a '57-'62 Thriftmaster 235 backed by a 3-speed tranny. The J engine was used in the '57 34,35,3700 series, the '58-'59 31,32,36,3800 series, and the '60-'62 C10-20-30 series trucks. The casting date code on the block would pinpoint the year.
