I might be tempted to pull it out with a rigged-up slide hammer. I've made 'em up from scratch here at the garage from scrap. In this case, I might use a piece of 3/8" round rod and a chunk of steel or iron to use as a slider, anything laying around that weighs maybe 5 lbs or so. weld a stop on one end of the round rod, slide the hammer onto the rod and weld the other end to the crossmember in the center of the dent. If it didn't want to move cold, I might apply a little persuasion from a blue-tip wrench applied around the outside of the dent. If the dent doesn't pull out square with the rod at that position on the member, grind the rod off and weld it in the next apparent depression. When it's pulled out to your satisfaction, grind off the rod and finish with a light coat of body filler if you feel it necessary.