The 640 Math SAT score is a huge red flag for MIT. I would be inclined to just forget about it. I was a student at MIT back when there was no SAT preparation. Probably about half of the students had 800 on the math SAT anyway, most were not very far from this, and we still found MIT to be quite challenging.
MIT also has no merit based aid. You would need quite a bit of need based aid to get the cost down to $50,000 per year.
I think that this also applies to Stanford (both points) but I was a graduate student there and did not ever ask anyone about SAT scores. I did however have a good friend when I was there who had done their undergrad at UNC CH, which I think is worth looking at seriously.