I've been able to pass probation tests by drinking a combination of cranberry juice, water, and gatorade (see: "how to pass a drug test" thread). You don't have to quit, but it is the only way to know for sure.
Look into
Gov's drug testing posts. He never failed a test in like 5 years of testing, yet, he smoked everyday.
edit - but yeah, being overweight really does hurt you. The metabolite is stored in the fat cell.
2 - when I say "you don't have to quit." I mean you don't have to quit, but it's best to not smoke for a week or so.