Im no engineer, so this is worth what it cost.
That design is not ideal, but very similar to how I built mine, much better if you can make the tubes straight line from mount to firewall attach. Then lighter tubes can be used.
Those tube you highlighted are under tension so 5/8 tube is fine. Basically, the thinner the wall thickness you use the more critical the welds. So choose thickness on your welding skill. For long tubes under compression, go up a size or so to help with bending load.
Best advice I got was to go around the airport looking at what different certified aircraft get away with. They never look strong enough but seem to do the job.