The cast is defined by ISO/IEC 9899:1990 §6.3.2.3.1, since block is a pointer to void, and struct header_t is an object type:

"A pointer to void may be converted to or from a pointer to any incomplete or object type."

The subtraction is defined by §6.5.6.8, provided that block points to an element of a large enough array object:

"When an expression that has integer type is added to or subtracted from a pointer, the result has the type of the pointer operand. If the pointer operand points to an element of an array object, and the array is large enough, the result points to an element offset from the original element such that the difference of the subscripts of the resulting and original array elements equals the integer expression."

(There's similar text in other versions of the C standard.)