The Frame on my trials car(s) starts with a Type 3 VW sub-frame/torsion bar housing. The roll bar is regulation 12g 1 3/4" mild steel tube & the rest is 16g 5/8" mild steel tube. I use square section tube because it is easier to match up to the next bit without having to profile it. Everything is bronzed together so it looks neater, and if any cracks occur, it is easier to repair on site. Square tube is also just a bit easier to straighten.

On later cars I've used 3/4" tube as the 5/8" stuff probably bends just a bit too easily (but only when I lean on trees!).

There's no hard & fast rule about where to attach the frame to the roll bar. In this instance the triangulation applied here is nice and widely based and has stood the test of time anyway. The regs state that the rollbar must be braced, which is effectively what I've done here. The rearward bracing is the strut that holds the motor & gearbox up.