I am building the Arbor for my wedding, and I have never built anything this large before. It will be 8' tall, 6' wide and top heavy. I want to keep the crossbeam at the bottom simple as I plan to make this into a garden porch swing. I was planning to use a simple Mortis and Tennon T-junction for both legs. How wide (front to back) should the feet be to keep the structure from tipping forwards or backwards