SpawnHeightVariance, and SpawnHeightOffset appear to only work if you use "HighestSolidBlock" as spawn Settings.
The First one is a Randomizer. That moves it from 0 to -3 Blocks. The Other is a Hard offset which will move the Object. I cant tell you what values you need to use, because I dont know the Coordinates of the Objects blocks.
As for the object itself, it appears that your Center Of the Object is way off. You should use a WoodenHoe to manually set the Center of the Object correctly. Or you could use my BulkEdit.