It would be less complicated to not have a classic car at all, so if that’s your argument why bother?
Part of having a classic car is working on it. While there’s nothing wrong with a /6, upgrading the car to a V8 will improve it’s appeal and value. And make it more fun to drive IMO, which is the whole point of having a classic to begin with. A 4 door /6 car is pretty much at the bottom of the value-for-originality scale. They’re literally donor cars for most folks, so anything that improves the value and drivability of the car is a win in my book. Why discourage that?