How much chassis stiffening needed?

Just seen a video yesterday with a 64', 1,300 HP Comet, missing everything you mention here. $204,000 and 23 years invested and only a lap belt. First time out he rear ended a SUV at a red light, due to brake failure. Both occupants were severely injured. Had they had a harness they would of been fine injury-wise, financially, no.
The car had harnesses in it, the Idiots weren't wearing them.