It's normally easier to just use deck mud. You may have difficulty getting the slope proper when using cutoffs, but other than that, if you thinset them in place, it should work as long as your slope is correct. By cutting the pan asymmetrically, you now will have an uneven perimeter around the shower. Functionally, it will work fine, aesthetically, you may not like it. It's best, when you can't modify a preformed pan symmetrically, to make one out of deckmud.