There should be a config option for all three altar blocks in Stalwart Dungeons to determine what item can be used on them to summon their respective bosses.
I think something like this would especially be useful for modpacks and to make it easier to summon your desired boss without having to fight another boss to get the item needed to summon them, being the Nether Star from the Wither.
@calm glen, you have reached the level 49!