I agree with Matt, with a 400 you may have some misunderstanding of the particulars of how the test functions and what the different question types are about.
You do not have a lot of time, but at least you are saying 650 and not 750 so it may be possible.
I recommend that you take some time to learn the five question types and how to approach them. What is your methodology when you see a data sufficiency? What are the steps you take when you see a sentence correction?
Once you understand generally what to do focus on your strengths within each area. This may seem a little counter-intuitive but with a goal of 650 you can miss about 1 out of every 3 questions. That means you need to get 2/3 right. Focus on getting proficient in the areas that you can - if you still have a few weaknesses, like say coordinate geometry, these will not kill you on test day and you can skip these problems and devote time to others.
Does this make sense? You cannot become an expert at everything in 6 weeks! But maybe you can at enough things.
Keep in mind this advice is based on your limited time frame and your target score...
