You could cut the shaft down but...
1) Will the rest hold up a heavy fish arrow? A fish arrow is usually at least 1500 grains.
2) The more arrowweight you have the better the pentration. Unless my arrows get damaged I shoot them full length. (and I have a short draw length) That extra weight helps drill those fish in deep water.
3) Depending who makes the arrow, the spine weights are all over the board on fiberglass arrows. Cutting it down may make it fly better or worse depending on your set up. (just something to keep in mind)