Has anybody done this before?

I think it is only wrong if you are trying to pass it off as something it is not. If you just like the looks of the 340 cars and want your car to look like one and are open that it is an original 318 car...I say go for it. Most people modify their cars to suit their tastes, very few cars are perfectly 100 % original. I Installed a 426 crate hemi in my 67 coronet R/T convertible a few years ago. When I showed the car, I had a display sign with it that stated it was a clone.