Right, this is extremely similar to what happened with my rear axle, only mine was much worse.
Yes, obvioucly, you need to change the bolt, then use the bolt to work out where the other mount part needs to be. Bash the cr@p out of it to get it as near as poss then file the hole in it until it is in exactly the right position.
Once there, weld a substantial penny washer into place on the mount.
Once this was done, my new axle located perfectly to the body and when I took it to an alignement shop, that wheel was better positioned than all the others.
Here is the wheel position before work was carried out.
I think you'll agree this was a lot worse than 20 mm!
Here is the displaced position of the bent bolt and it's mount
Most of the displacement was taken up by the bend in the axle.
As you can see, it bent at the point where it was attached by the radius arm.
So it pulled on the radius arm which in turn bent the bolt at the chassis and moved the mounting point. So the damage to the mounting point was not too bad. With a replacement axle and the work as I described to the chassis mounting point all is now well.
So it may be worth you looking at your axle to be sure there is no bend in it. Otherwise the type of work I have described will repair the mounting point and make it solid enough for a good repair.
What I'm thinking of doing is as the chassis looks good is cut the bolt out, order a new bolt and brush. Weld a 4mm plate over the old hole and drill a new hole through the two in the right place? Then using a few triangles of the 4mm plate weld in a few supports so it cannot move any further. What do you lot think, I'm interested to hear opinions as this is only an option.
This will be more than enough.
This is actually more than I got done so welding the plate/washer will be all you need to do. The main thing supporting the bolt is the tube, the rest is only there to hold the bolt in position in relation to the radius arm. The bolt is what takes the strain. If you try to weld 4 mm plate all over the place you will need a longer bolt!! You do not need to make it stronger than the original.
Best of luck!