Another boat question

Should have had it welded when I had the cracks done. I’m not so sure that may have caused my issues on the inside plate of the transom. image.jpg
But I didn’t want something like that going on where the outboard sits flush on the outside. The motor is tight on my “shims” and I used the sealer on all 4 sides of the plates. The transom is hollow! One 0.125” plate outside and a double layer inside.

3M makes two marine sealers 5200 is semi permanent but still flexible. I think 4200 is more like a caulking??