Upper shock mount broke clear off. Flippin' unbelievable!

At least with flying, you'd be reasonably assured when people worked on your aircraft, they'd be certified to do so. Trailer frame welders, not so much. I see nothing wrong with your design as described. I'd only suggest a cylindrical spacer around the bolt between the 3" plate and the main frame web. That way when all bolted up tight, some of the stress would be transferred to the frame web instead of all of the stress being on the 3" plate.