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1-1/2" square 12 gauge steel tube for the main frame. What you see in the pics is 560 lbs so far.
That's too much, basically. Start with 16 gauge x 1.25" for the main rails and 1" for the rest. You'll need cross tubes at each of the bends in the main rails. Early on you need to draw in the steering rack and coilover mounts. If you want inboard suspension that's an early area you need to flesh out too. Are you going to use trailing arms for the rear suspension? Those details will affect the frame a lot.
If you are going to register this for street use, I think you will be happier to go for a two seater. Unless you are planning for a solitary life without friends or lovers.
Being unable to even hold a grocery bag will limit your use of the car and they are a lot of work to make. The extra seat will not really slow you down....Changing those tubes in the drawing from 12 gauge to 16 gauge will save more than enough weight to make it a two seater.
I used "Grape" to do my tube frame FEA. It's not that hard to learn to use. Grape does not mesh the frame, it uses simple equations that describe the stress and strain of tubing to do it's work.