Are you talking about the blocks in the rear. Why don't you just take them out if that is all your after? Those blocks are about $10 or less if you just want to buy shorter ones. However, there are no blocks in the front...Do you just want to lower the rear?
Any type of cutting will heat up the object and cause it to be weaker. Even if you managed to shorten them with out much heat used, they would still loose their strength.
Buy a Kit!!