Acoustic Guitar Construction

Why so big? 1/8" depth would be just as effective. Those splice tenons add zero strength to the joint, maybe compromise the neck block? They are merely locators, which leads right to -- have two bolts (takes care of the perpendicular) and also adds real strength, countering the lever action of the neck string tension. Oh wait! That's been done before.

The splice tenons are glued into the heel with HOT HIDE GLUE , boys and girls, so there is nothing to worry about.
Just kidding :-)

More importantly, I did find a pic of the early Taylor butt-joint system. Man it is simple, and worked great. Got the pic from:
http://www.frets.com/FretsPages/Luthier ... reset.html

My experimental "lab guitar" constructed 1980 ish, has that style joint retrofitted from a dovetail, still going strong for 8 or 10 years now. On that one the FB extension is glued to the top cut off at the 14th the fourteenth fret covers the seam at the slot. Also I have a 1/2" dowel running all the way thru the heel. You can see it in the pixs

Ok. A hardwood dowel to hold the inserts better?

Correct

What in general would be the recommended minimum thickness of the heel on a butt-joint? Here is a picture of a sweet heel from a gypsy guitar, but of course they are traditionally dovetail joints.
For a butt joint, the heel looks a little thin? Is there a good minimum thickness?