Yes, that's it exactly! A lot is done to the trucks aftermarket, which is not always how they came from the factory.
Examples:
I bought a truck from a wrecking yard that had a D200 cab swapped onto a D300 frame. Took a awhile to figure that one out.
A local guy here has a 67 W300 with 70 W300 axles and 383/mechanical clutch swapped in.
I put GM 10 lug 20" split rim dually wheels on my 62 W300 with 6 lug axles.
TONS of trucks have weird mods, but some take more work than others.
Even so, there are some things that can't happen w/o changing something else too. That's why no D100 has split rims, cuz no 5 lug D100 wheel was ever made that way.
Plastic trucks suck...
Iron trucks rule...