Trouble fitting outer sill on 64 S1 OTS

Since the outer sill has a U-shaped flange along 3/4 of the panel, it appears that I have to engage this with the lower edge of the cowl as well as the lower edge of the rear quarter panel, then rotate it into position. My problem is that the front 18" or so of the outer sill has a vertical flange which interferes with the inner sill and prevents me from rotating it fully into position. Is there a trick?
Thanks,
Al