Remember that the steerer on the threaded fork only needs to extend beyond the head tube to accommodate the threading of the headset cone, the washer, and the lock ring. The steerer on a threadless fork needs to extend far enough beyond the head tube to accommodate the top cap of the headset, the stem clamp, and as many spacers as you wish to use to raise the handlebar. It appears that if you cut your replacement fork at the end of the threading you might no't have enough left for the stem.