DR Tom, With the system I used there is no need for any "tacking." The dovetail slot in the bottom of the upper rail holds the spindle 4 way dovetail and the filler that fits that slot and has a 15 degree on each end to match the top of the spindle dovetail. That results in all 4 sides of the spindle dovetail being engaged. The entire mess is simply slid in, one after the other until the last piece. That is a very short piece of the DT filler that has been glued into each end to trap all the rest of the parts. The reason the last filler piece is very short is to maximize the remaining slot so that a piece of filler can fit there and be slid into the matching porch column DT notch. That is the only piece requiring any mechanical fastener, a SS screw to keep it in the column.
This was one of those exercises in design that would drive a normal fabricator to drink.
I'm an architect by training but chose instead to develop an architectural millwork shop. My shop will take on projects that most woodworking people (with any brains) would pass on. We do lots of curved work in cabinets, moldings, Corian, plastics and veneers. The Corian & plastics are thermoformed in a 3'x8' oven. We have 3 very skilled CAD people. The shop is equipped with computer controlled tools driven by some very slick software. By the time the parts get to the clamps or the benches they are ready to be assembled with no at-the-bench fitting required. All the pilot holes for the hardware are there so no measuring is required. The primary method of construction is doweling with Confirmat screws used for other (oddball things.) I'm retired now so just do whatever I want. I have full use of the facility but I'm really poor at CAD work. I have a corner of the shop that I do metal machining and I do metal casting out back. Us old guys need to keep finding new challenges or end up couch potatoes. This weekend's project was chemical etching aluminum. Worked quite well. Always push your limits! Until you fail you don't know where they are.