The commercial chassis gets cut where it widens out right near the front cab mount is. You can see a vertical slot on the outside of the chassis, that is cleaned out, & the car chassis is stitched through that slot.
I reckon there would be a repair procedure around, someone may have a copy of.
This was a common repair on tonners in the 80s & 90s, I was involved in a few over the years. Guys bought front end damaged ones, & put them back on the road. Would buy a sedan out of the paper on the weekend, (we would usually buy prems or done up cars, so we could sell off all the bits) strip the chassis down, & take the ute/tonner to panel shop (with the cab off, & bare replacement chassis, & for a bit of cash, they would get it done after work within a couple days.
The ute would be back on the road next week.
Never actually saw it being done, but I'm sure the car rails were cut, & only part was tied in

Cheers
Max