Some people like myself do care about date codes, because cars are only original once, and I for one do not want to own a Taiwanese made Mustang.
Also, if you have a high value car like a cobra jet or boss 302 or boss 429 or shelby etc, i guarantee you with 100% certainty that nearly every single potential buyer is going to care a lot about whether it has the original sheet metal with the date codes on it or not. I know this for a fact because I have been repairing, restoring, and selling these for a living for 45 years, and if you have 2 mustangs that are identical except that one has all original sheet metal and the other has mostly crappy taiwan sheet metal, the taiwan car will be worth much, much less than the original sheet metal one, unless of course you find someone that just happens to prefer crappy taiwan sheet metal with no date codes that is mig welded to a car, instead of original ford sheet metal with date codes that has the factory spot welds etc.
Also, if you want to replicate the original spot welds, you will have a hard time doing it yourself unless you have a spot welder with the right tips like the one I use, and this is only going to cost you even more additional work and time, as opposed to just replacing the lower section if that is all that is needed.
The 68 mustang dynacorn had at the sema show several years ago was nice, but it cost them a ton of money to have someone make all their aftermarket pieces line up together.