18" wheels heavier than stock?

It should be pointed out that the 15 x 7 cop wheel is considerably heavier than any A-body stock wheel (14 x 4.5 or 14 x 5.5). So your comparison isn't really against "stock", it's big aluminum wheel against big steel wheel.

The other point I would make is that as the wheel diameter increases, the profile of the tire decreases (in order to maintain approximately the same overall diameter) -- the effect of this is essentially replacing air and rubber with aluminum. Which is heavier? Moreover, you are adding the weight out near the circumference of the combo, where it has a much greater effect due to rotational inertia. Forged wheels are much stronger and have the potential to be much lighter, so they can compensate for this issue. But they are also much more expensive.

Lots of factors to consider.