The outcome would be a little different without that X-frame, not wildly different, but...
That '59 Chevy might as well have hit a tree. The X-frame which was under that car might have had good (for the time) torsional integrity, BUT nothing near what GM later used. Even on a '68 Buick, look for structure in front of the radiator! NONE!! A flimsy chrome bumper attached to two frame horns, NO real structure until you get to the cowl. Somewhat typical of GM designs until the middle '70s or so, by observation.
In the early '80s, I took a body order to a customer, for a '81 (or so) Olds Delta 88 2dr. That platform was GM's first to use computer analysis for design/structure. The young girl had seen a friend at the local Dairy Queen and turned into the parking lot, from the inside lane of a US highways in a more rural town. Only problem was that a high patrol trooper was in his state-issued patrol car. Whoops! When I got to the body shop, they had the full interior out of the car, primered metal everywhere. Which showed a very evident wrinkle in the floor pan from the rh cowl/door hinge area to the drive shaft tunnel, near the back of where the seat would normally be. I was shocked! Considering how many front end or rear end crashed I'd seen those cars survive very well, fixable with bolt-on parts.
I ended up taking back the rh door assy and hinges, as the metal they mounted to on the cowl had just been broken loose. Repositioning those panels, re-welding, put everything back where it needed to be. But that wrinkle across the rh floorpan/footwell area was troubling.
Over the next decade or so, the lack of lateral side-impact protection on that body series was graphically indicated, many times. But when the federal side impact standards were improved in the later '90s, GM cars became light years better in that respect, by observation.
There was one chain-reaction accident I saw one Friday night on "the main drag". A really nice '67 Nova had stopped for a car in front of it, but the '66 Fury behind it didn't. That particular Fury used to belong to our family's dentist, a normal Fury III 4-dr sedan. It has apparently been purchased by an owner that lived "outside of town", from the amount of accumulated dirt/mud that was knocked loose by the impact. The beautiful Nova (had been around town for many years, lovingly maintained by a few careful owners (it did have a 327HP factory engine in it). The quarters on the Nova were buckled a bit. The then-current owner was an amateur body builder. You could hear his anger and frustration in the sound of the short-handled sledge hammer as the impacts reverberated off of the buildings. The young girl in the Plymouth watched. The Plymouth didn't even have a bend in its grille, much less the bumper. No leaks, either.
A good body shop and a frame machine could probably have saved the Nova's factory sheet metal, but I suspect the car got parked somewhere, out of sight. Once the owner git the body away from the rear tires.
Every look under an Imperial? By comparison, those cars seemed to be as stout as any 1-ton pickup built when the particular model year of Imperial was built. In the middle '80s or so, a man left the DFW airport and headed west. Only problem was that an old across-the-freeway runway was still in place. The rh lane had to merge into the land to the left, as that particular lane ended at the "concrete wall" of the runway's support berm. A well-known situation/hazard. Apparently the driver/man had a medical issue as he didn't merge to the left . . . at probably about 50mph (considering time/distance/traffic patterns). The fuselage Imperial he was in hit the concrete wall head on. I happened by, on the other side of traffic before they had the car removed. The main thing was that the body was bowed, front to rear. Normal front end damage, it appeared, probably some wrinkles in the roof? But no broken glass. It didn't just crumple, only bowed with the rear bumper higher than normal.
I read later that the man died AT the hospital, from internal injuries (I believe). Air bags might have saved him? When they hauled the car off, all of its sheet metal was still attached, meaning that the Jaws of Life were not needed to get him out.
IT was funny that when Ford put some crumple zones into the front frame horns on their '69 Galaxies, they worked in energy absorption. So well that GM was quick to see how they did it. LOL
As for the "strength of UniBody", as Chrysler did it, an article in one of the Motor Trend "annuals" of new cars, about 1969 or so, mentioned that a UniBody-type car's body would need about 150% of the force needed to permanently deform a body/frame car. By observation, Chrysler did the best UniBody-type cars, outside of the '58-'66(?) Lincolns and T-birds, by observation. The Square Birds, as I found out years later, had enough "guts" that people would make convertibles out of coupes, with no body problems.
When we got out '66 Newport Town Sedan in late '66, I was impressed as all of the switches/knobs on the instrument panel were all recessed well under the edge of the padded dash. The '67s were not quite that good, but the '68s (I believe) added some lower padding to the panel.
As a side note, in the middle '70s I drove by a metro-area Chevy dealer. On their lawn was a '68 Chrysler T&C wagon. The dealership was hosting a "car bash" for charity. They had the Chrysler wagon and another non-GM car. The young college "studs" were seeking to wield that sledge hammer and show off. I saw one take a hit on the Chrysler's hood. He looked amazed as hot the hammer just bounced off, leaving a minor dent. I laughed and smiled! Later that afternoon, the Chrysler was still salvageable, but the other car didn't fare as well.
Years later, our Mopar club had a similar activity, but with a '75 Buick Electra. Didn't take long for that Buick to be complete decimated. ALL of the Sheetmetal was severely dimpled/smashed. Anybody that wanted to take out any of the glass, that was a "cheap shot". LOL
By observation, the '65-'68 Chrysler C-body cars were some of the most solid cars Chrysler had ever designed/built, period. Everything was either bolted solid or welded. Which was probably the reason they had more road noise than a similar GM or Ford "frame-isolated" product. If the dealer didn't order the factory undercoat, THEN the "tin can" reference might be operative. Buy those GM cars would be nice and quiet on smooth/solid roads, but on a gravel or dirt road, when the rocks would fly up and hit the un-undercoated bare metal of the floor pan, not the underdoated fender liners/wheel houses, then you realized what you were driving on really quickly.
As good with road noise attenuation as the "Torsion Quiet Ride" C-bodies were, they just didn't have the same "brick=like" feel of the '65-'68 C-bodies to me. Still FAR better than anything GM built, though, in strength, back then. Just like a friend's CHallenger T/A on a rough road (only heard suspension bushings flexing a bit) compared to my '77 Camaro "rattle trap", by comparison. As if GM never really knew how to build a good unit-body car!
Sorry for the length. Just my observations of many decades . . .
CBODY67