[Rodger Knox]

There's no real reason for the 14th fret body join, other than it is typical for acoustic guitars. It also works out to put the bridge in a good spot with this body size and your scale length (which is one reason I recommended that size). You can certainly adjust whatever you like to make it work with the neck you have (or get).

I intended the top and back to be flush with the outside of the box, as is typical in acoustic construction. There is a 1x4 cut to fit below the stack, but it might be better to have that solid, since the neck bolts will be going in through the back.

I'd put a 1 1/2"x6"x 1/8" maple bridgeplate under the bridge, and no other internal bracing. You could do more, but I don't think it's necessary for that size top if it's 1/8" thick spruce. The bracing depends on the thickness and stiffness of the top wood, and with 1/8" thickness almost any piece of spruce will be adequate.