http://www.northerntool.com/images/p...s/55868_lg.jpg
This is an example but you can find a threaded blind rivet tool in sizes much larger and specialized fasteners with splined edges in whatever alloy you need.
This will allow you to use a machine screw and a larger load area.
I have used this tool for applications such as yours with great success over the years. Boat builders are fond of attatching seat and hatch hinge screws into thin laminate with self tappers. They will always pull loose and tear. This fixes it forever.
I would spend 25$ and try this before cutting up your deck.
Good luck.