Its not illegal to weld chassis, we get chassis extensions and wheel base mods done regularly and they are not riveted. Weld at a 45 deg angle with a reinforcing section inside going through the welded section.
This is part of the heavy vehicle modification regs. I am not sure about light vehicle regs, but if we can weld a truck chassis which will be rated to 23 tonne GVM I doubt they would have any issues with a welded chassis on a car.
I do remember years ago a guy repairing a crashed falcon told the department he had used a new bodyshell and they wouldnt approve the repair, so he went back and told a different guy he had replaced the front chassis rails and a full long rear cut and they approved it. Problem was, it's illegal to remove a body plate and affix it to another vehicle, which is what he technically did for the body shell, so he told them it was 3 cars made into 1 and that was fine. Red tape technical bullshit.
JD