If you can do a box step with 200lbs on your shoulders you don’t need to do this exercise. You have more than adequate general strength for any sort of mountain type goal I can imagine. At this stage you might consider transferring this general strength into an even more sport specific activity. I’d suggest Hill Sprints as described on our book on pages 230 and 231. Keep in mind that you need a very steep (30-50% gradient works best) hill or a set of stairs for this maximum power effort. With your strength you might need to use your weight vest. Another option if you gym has a sled to push or pull is to use that for 8-10 second max sprints. These exercises will build more specific leg power for climbing steeply up hill.