As I'm looking at the last two photos, it occurs to me that you might not want to bother with the 59 parts for the A-pillar, and just scratch build them. At a glance they are similar, but that windshield appears to wrap back even further on the top than a 59 Impala. Another point is that you need a vertical A-pillar. The 59 Chevy angles forward before curving back at the top. Look at the side view you printed to see this. Just curious, but how are you determining the width of the car?
Hope my observations are helpful, I really want to see you do this right! This is a great project to take on - GOOOD LUCK!!