[Ned Milburn]

Yes, that looks pretty disconcerting.

You can check for grain run-out by splitting a corner of the wood - just a half inch or so - to see the direction of the split. If the split runs parallel to the top or close to it, you are looking good. If it is a noticeable angle, you know you've got some grain run out.

It must be remembered, however, that sometimes one edge of the wood will be a greater victim of grain run-out than the other edge. Hence, you should try to join the edges with the least amount of grain run-out to keep this portion of the halves in the center of the guitar, with the grain run-out towards the guitar edges.

Is your wood large enough to flip and use the discoloured section underneath the finger-board extension?? (Possibly not, since it looks like you've already cut to the shape of the top already.)

PS - I understand not wanting to redo something already done. I, in fact, have a perfectly fine, nearly finished, classical/flamenco neck in my shop that I thinned about .5mm thinner than I wanted, so it has now become a "demonstrator". Also, I've got a few tops hanging on my walls joined and thinned, but that I am not using. Two because of bad grain run-out, and one because I thinned it a bit too much when my thickness sander was new to me. Easy come, easy go. Tuition fees!! ;-)