You read my mind. That was the plan.If you get some M12 studding and 4 nuts you can thread it between the axle mounting tabs once you've made the cuts. Only cut the top and two sides of the inner arm to main frame member and leave the bottom welded and just bend it up. If the gap closes before it's bent far enough then run the cutting disc down the cut again. Repeat until it's bent enough. Once somewhere right the studding will go in and when the nuts are tight against both sides of both tabs the tabs they will ensure the tabs are vertical (at least when measured against each other). You'll need to eyeball it to ensure they are both in line with the rear. If the tabs are vertical the wheels will be.
For checking the angle, was going to do something similar. Mini level on the bolts.