The drivetrain design being done is good news. That means the battery, inverter, motor(s), and control electronics are completed. The design being close to finalized makes sense when looking back at the Model 3 reveal, because the demo cars from that event did seem like real cars rather than concepts.
6-8 weeks would put design completion at the end of June to early August. Typically, it takes about a year to go from design completion to actual production, which would get Model 3 to a Q3 '17 initial delivery date. Beta testing/tweaks, setting up supply chains, and gearing up the production line for an order of magnitude scale-up will be an unprecedented challenge in company history.
This is all extraordinarily ambitious.