I've put 240 front turn signals on a few 280s. You don't need 240 headlight extensions.... those are the same unless you have the fiberglass ones like noted in the thread... even then I'm not sure what would be different about fiberglass extensions. The valence corners and center pieces below the headlight extensions are what's different between the cars. The shapes of the valence pieces are definitely different between 240s and 280s due to the car's structure behind these pieces. There's more structure behind the valence pieces on 280 than on a 240. One difference is the radiator core supports, but there are many other differences, as well. Because of these differences, the stock 240 valence pieces are not just a simple bolt-on to a 280.
The easiest way to get 240 turn signals on a 280 is with an aftermarket front 240z spoiler. Most will bolt on with no modifications required. The Kaminari front spoiler that I like to use is an exception, though, and requires some cutting to make it work.
Here's a pic of the last 280 I converted to have 240 front bits: