340 or 383 for 1968 Dart

I have a question I don't know the answer to:
How much more does a 400 with aluminum heads weigh, that an iron head 340?
The comparisons I've seen if you also go aluminum water pump and water pump housing is only about 15lbs and you have a front position distributor, external oil pump with easy accessible oil filter and the dry intake manifold no mess of antifreeze when you work on it. All that 340 is a mean machine and roomy engine bay. Either way has some benefits.