If your average for the blocks is a 58% (68, 58, 48%) then it is estimated by medfriends.org that you would get a 177 which is a failing score. I would highly recommend you buy one of the $45 NBME exams and see how you score on that. Those are usually a good indication of how you will do.
It is concerning to me that your scores progressively went down significantly. That was only 3 blocks as well. Test day is 8 blocks. Was fatigue a factor for the 2nd and 3rd blocks?
Unless I have misunderstood your post, I do not think it is possible for you to get to a 250. It sounds like you are well into your studying. Since you have already started studying, to be able to get to the 250 mark you would need to be scoring much higher than you currently are. You just can't make up 70+ points in 3 weeks.
On the free150, you need to get 79% right to get a 237 and 85.3% right to get a 250. 58% is a far cry from that. At this point, you should take an NBME to see if the free 150 was a fluke or not. If it was not a fluke, you need to set your sights on something more realistic.