I would be very weary of 1" spacing with a mega tree, because
1. That density would be way too bright even with brightness turned down in the controller, just because of the density.
2. You will not have anything close to 1" spacing horizontally. I believe it is better to have the horizontal and vertical spacing close to the same. A mega tree has variable spacing so it would be best to match something around the middle height of the tree. If you end up with 6" horizontal spacing at the bottom then you have roughly 3" spacing in the middle of the tree. I don't use much in the way of images or text on my 3 pixel trees, but with 3" spacing I think they look fine. YMMV.
3. 1" spacing is more costly, more time consuming to build, requires more power, more controller committment, etc, etc. But it is your commitment and your tree.
I would try 2" spacing first. Just my 2 cents.