Is there a block at the end of the slot that needs to be removed to allow both the tail and the horizontal stab to be installed? It sure looks like there is not enough room for the stab to slide in either, and from the looks of the tail I'd think the horizontal stab should be installed first (at least that's the way I've always done it). The way the tail sits you'll need to cut a notch on the lower aft side of it. One technique of rounding off blocks that sit above the horizontal is to put a small sacrificial piece of wood in there that can support the blocks as they are tapered and sanded to shape without the horizontal or tail having to be installed. I'm a think'in that's what your looking at (but I could be wrong - happens on rare occasions ).

Check to see if that isn't a removable block there. Try installing the horizontal stab (slide it in as far as it will go, or center it up if it goes all the way in) and take a picture of that - that will let us have a better idea of how things are fitting.

Hogflyer