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I have a stack of pretty nice Italian spruce, however, they're all P size only.

What is the proper method of adding the wings to widen the lower bout?

I haven't done one myself, but theoretically the runout should be best matched if you swap the waist cutoffs. So glue the treble side waist cutoff to the bass side lower bout, and vice versa.

Flipping it over across the grain inverts the runout, but flipping along the grain doesn't. And to get the straight edges together, you have to do a horizontal flip (or rotate 180 degrees, which is equivalent to horizontal flip followed by vertical flip). So start with the opposite runout and you can't lose

So what are you trying to do?
Squeeze a 14" lower bout out of a 13" top or make a D size out of a 12" top.

One - you just use the upper bout cutoffs oriented to keep the runout the same.....
The other will require gluing a second book matched top set to the first.

Or make 3 piece tops without a center seam