I am going to drink tons of water and take regular doses of Niacin
Niacin is an old urban myth. It does nothing to pass a U/A. Its the waterloading factor, and not the niacin itself, that allows people to pass.

Drinking a bunch of fluids days before the U/A is of no help. It may make you urinate frequently, but the rate in which THC metabolites are deposited in the bladder from the bloodstream remains unaffected. Therefore, waterloading is only effective on the day of the U/A.