You don't need to drink that much water man.. Just drink a normal amount and stay hydrated. You stand an excellent chance of passing. I smoked a gram or so a day for an entire year straight and passed a drug screen after 18 days and we have incredibly similar builds (I was 5'11 145 pounds at the time).

In the meantime keep excersizing and eat a high fiber diet (use metamucil). It is important to note that 3-5 days prior to your test stop excercising completely and eat fatty foods and just be lazy as hell.

If you want to alleviate some of your concerns buy a home test and test yourself a few days before your real one. If you pass then hell yeah, but if you come up dirty on your home test follow the dilution method that is stickied at the top of the drug testing forum. If i didn't make anything clear or you have some questions feel free to ask.

Remember, you don't need to chug THAT much water

EDIT: Remember now.. you are on probation so if you are going to opt to using the dilution method make sure you do it right. For probation a negative dilute is the equivalent of pissing dirty so do some practice runs before you take your test for real. Preparation is key.