Springs should be angled outwards, the further the better but 30 degrees at full compression is the recomended maximum. I would suggest boxing at least the back of the frame, but first you may want to narrow the rear of the frame and move the top mounting locations inboard to get the desired angle on the shocks. I would recomend boxing the frame it's entire length with 10 ga. steel. One other thing, those rivets may have been adequate in the low horsepower original car, but with modern horsepower and traction the rivets should be replaced with welded joints. JMO