I had the same problem on our '88 745 GLE. I removed the aluminum bumper and was going to cut out the rot and have 1/4" plates welded in. That aluminum is HARD!

I noticed that there are ribs inside the plastic cover, so I had the plates welded on the face of the bumper and drilled holes for new bumper bolts.

I then cut the ribs from the inside of the plastic cover where the plates were located. I inserted thin pieces of hard wood from a cheap pallett behind the aluminum bumper to make up for the missing metal.

When I was done, there were small bumps where the bolts were, but they did not look bad at all.

The aluminum was scrap, so I only had to pay $20 for a nice solid repair.
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96 855R, 95 855,854, 90 744 Ti - 340,000 Volvo miles put on 7 bricks