No problem cutting bolts down, I do it all the time. It just means you lose the plating on the end, so in some situations, it may rust more than the original bolt.
Looking at the picture of your bike and the kit, I take it two bolts go in to each pivot from opposite sides?
If so, you will have to cut it down so as not to bottom out on the opposite bolt.
Bolts come in different grades though.
If you look on the head of one of your existing bolts, there should be a number.
8.8 is standard high tensile. If it’s not marked, you can generally assume it is this grade.
If it’s marked 10.9 or 12.9, then you really ought to replace it with a similarly marked bolt, as these are greater tensile strength.