[Kube]

This issue is both interesting and simultaneously almost a non-issue for many of our cars.
Have any of you really paid attention to how the seats are bolted in? Do you really believe in a frontal crash the seat is not going to be yanked out of it's mountings? If so, think again.
In 1939, the wagon seat is bolted directly to the sheet metal floor pan, the '39 convertible is a tad better having a reinforcement piece of sheet metal in that area. So, not one but two pieces of sheet metal that the seat is bolted to and subsequently ripped from.
Belts in these old Fords are mostly for "piece of mind" as the cars themselves simply were not engineered for (much) safety.
A tiny lesson in physics... "a body in motion tends to stay in motion".
In our Fords, that equates to YOU and everything else behind the point of impact keeps moving forward until it has something (a tree perhaps) that will stop it.
Arms and legs are dislocated at their joints, that "cool" "beanie baby" your wife put atop the package tray just became a large bullet traveling toward the back of your head at the speed of the crash...sure, it's soft but at 55MPH, it'll break your neck...

I first hand witnessed a fellow put his teeth through the steel dash on a '53 Chevy. Looked like a perfect copy of his teeth. That's physics my friends...