OK folks...so I'm at the point of fitting the horizontal stab and getting the attach holes drilled. I did the holes between the rear spar and the vertical attach bars tonight - those are pretty much a piece of cake after checking level and alignment about 400 times. The holes in the forward splice angle, though, I'm spending a whole lot of time reasoning things out before I drill, since it seems like this is an area where people frequently encounter problems.
The main thing I'm running into is that, as far as I can tell, it is 100% impossible to drill these holes and maintain "proper" edge distance (that is, 2x from the center of the hole). This seems to be a fairly well-known issue, and digging through past discussions indicates that a) the mothership knows this and reassured builders frequently and b) since the bolts are loaded axially (vs in shear), edge distance is less critical.
In an effort to be super thorough with positioning these holes - as opposed to simply taking the measurements from the plans as gospel truth - I did a whole lot of drawing on the aft deck with a Sharpie. With a little work in Photoshop for some colored overlays, we can end up with a nice little visual representation of all the pertinent pieces (I've omitted the spacers since they don't seem likely to have edge distance issues):
The real fun here comes in dealing with the overlap between the longerons and the angle riveted to the bulkhead. The most critical point in terms of edge distance seems to revolve around the lateral position of the outboard bolt holes. The important edges here are the inboard edges of the longerons, and the outboard ends of the bulkhead angle. It stands to reason, to me, that the best way to position this hole laterally is right in the center of the overlap of these two pieces, since this maximizes the edge distance on both pieces.
Hopefully this close-up annotated view makes this a bit more clear (note that I've removed the depiction of the stab attach angle since it's not critical for this particular case):
Note that the total overlap of the longeron and bulkhead angle is ~5/8", which in turn implies that the best edge distance I can get on both pieces is 5/16" if I really nail that centerline.
So the first question is: does this make some semblance of sense, and does my approach here seem reasonable?
Then we get into the question of how to actually go about drilling these holes. There's not a lot of detail in the construction manual here, so I suppose the naive way to do the drilling is to get the stab clamped into place, mark out the hole locations on the stab attach angle, and drill straight down through all the pieces. The main thing I worry about there is accurately getting the hole position marked on the angle, since most of the pertinent measurements need to be made relative to the longerons or other things that are beneath the stab. I suppose I could drill up from underneath, but I don't feel like that really solves any problems.
One thought that occurred to me was to drill #40 pilot holes just through the aft deck and attached pieces, then clamp the stab and spacers in place and drill that hole up from beneath, up to #30. Then I have to worry about being sure I drill square while probably working in an awkward position, likely with an angle drill.
So maybe the real question is: how did everybody go about marking and drilling these holes? After typing all this, I feel like it may be a rhetorical question to ask if I'm overthinking this. Which is why I'm typing all this tonight and then letting all this stew overnight before I go actually put holes in anything.
After all, I don't want my plane to end up in Vic Syracuse's hall-of-shame thread...