There are three (possibly 4) 289/302 spacers used in 1968: C8ZE-8546-D (marked C8ZE D) is the most common, 2.95 TOTAL length, measure the tip of the piece that orients the fan, to the pulley mounting surface. If you have air conditioning the number is C8ZE-8546-A (marked C8ZE A), same criteria for measuring. The other two are rare, extra cooling and engine rev L15.
Jim