Well made some nice progress today. Thanks for all the help
I didn't get stainless steel for the hold down but did use PVA tape as a barrier. I also use the doubler plate of aluminum, basically as per drawing. This 6" piece of Aluminum is stout it drops down below the angled hold down and will channel any water for its length below the plywood. I will do two rows of rivets through the new taller hold down plate. One as the originals, the other above them about 3 inches.
You can also see in picture the reenforced last cross member with some plate steel. The previous owner made a modification this makes it stronger than ever and steel allows some flex.
The 4th picture show the 6" aluminum doubler behind the hold down plate and above the yet to be welded plate steel over the x member.
Again thanks for the help. I hope there is not any glaring problems and the pictures helps someone else.