gimp - Recombine Scanned Images Without Seam

I am attempting to recombine the two halves of a large photograph from a flatbed scanner without leaving an obvious seam.

Each image is 7206 x 9450 (and is a separate layer below). There is about a 150 pixel overlap.

I can move each layer, confirming the position by toggling visibility. However, the rotation is a bit off. Rotating messes up the alignment of course and does not undo quickly.

Then, I got the idea of matching the alignment of the grey lines from the photograph being in the frame for so many years (see image). I hope that if I get the grey lines perfectly horizontal, then the layers will be rotated correctly. I would then be able to simply move one layer over the other and all features would match.

Questions:

1. Does GIMP have a feature in which I could select two points, then rotate the layer so a line between the points would be a straight line?



2. Would this approach likely remove the visible seam between the scanned images?