JH
Strength of the structure will be a large function of tube diameter, rather than massive wall thickness. Considering how small and light weight your car is and probably will continue to be, I would start thinking about 1 1/2" .083" wall, or 1 3/8" .095" wall. Of course with well designed triangulation perhaps the smaller dia with .083 wall thickness.
Don't forget to plan for where you "want" to be in a few years. Building is fun, but trust me, you don't want to have to rebuild every few years. Personally I would use the larger 1.5" tube.
The Ford car last year was 1.750 dia .120" wall, and would have been 2 inch around the driver if they had tooling for bending such massive stuff.
Have fun
Rick