An object denser than water will only displace its volume, so it would not matter if this was lead, copper, quartz, or gold. They all would displace the same amount of liquid given it has the same dimensions.
The purpose of displacing the water is merely to obtain an accurate measurement of the irregularly shaped supposed gold bar's volume. The density of the object being measured doesn't matter, as long as it's fully immersed.
E.g., if the gold bar displaces 14.6888 cc of water, then it should weigh 283.49384 grams, or 10 oz. On, the other hand if it's 95% tungsten, it will only weigh about 282.796 grams. Gold is 19.30 g/cc, whereas tungsten is only 19.25 g/cc.
I suppose you could make it more interesting by mixing in some osmium, which is 22.59 g/cc, but, unlike tungsten, osmium is expensive. Back in high school, a kid I knew had got his hands on the coin sorting mechanism of a coke machine. He found that it worked based on the magnetic properties of the coins, as well as their size and weight. For instance, a quarter-sized slug punched out of a lead sheet went straight down the reject path. However, he found that if he glued on a small fleck of iron foil, the hybrid slug would work just fine.
If given the same dimensions then they will not weigh the same.