Ok finally got a second to take some pictures and measurements for you. You are correct that the top rear rivets hole is just at a half above the frame. These are consistent from side to side . As is the 9inch measurement from the center of the front shackle mount on the frame to the lowest rivet. With the wheel off and the axle set on a block to simulate ride height. The axle is almost dead nuts with the 4 degree shim under springs and on top of the axle pad. My floor is level as well. So using a framing square rested on the level and butted up to the axle the angle looks to be about 50 degree from level. This makes sense in my mind. Hope it works for you
Honestly if you bolted the lower shock mount on the underside of the spring ,bolted up a shock to that, the angle of the lower mount will get you in the ball park of where you need to be angle wise. My upper mounts appear to me to be original location. And I base that solely on the fact that the rivets are still in place. And again are consistent from one side to the other. I hope this helps you locate them. If you use the shocks that I indicated you should be good.
The mounts are roughly 12 inches apart outside to outside.
