Look at the underside of a '66-'67 Fairlane if you want to see something almost scary.
They had basically nothing in the way of longitudinal reinforcement, from the firewall back to the rear spring eyes, except the rockers, which are smaller than our Mopars.
Almost no structure to support the trans or distribute loads.
A buddy of mine is a Ford guy, and has one of his Fairlanes up on a rack. I'd never really looked under one before.
He says that the quarters actually tend to split in the area under the roof C-pillars...
I've put frame connectors under A & B bodies, a Nova, and a Camaro; without exception, every one became more solid, handled better, hooked better, & rode better.
Even the doors open & close better.