I had been stuck for a few days, as I didn’t have the right drill bits to finish enlarging the holes on the rear spar. I did have a 1/4 inch drill, but I really wanted a reamer. I reached out to my friend Murlin who offered to lend me some bits while mine were in-route from Aircraft Spruce.
Using a #C drill bit, and a 1/4 inch reamer, I drilled a perfect hole in the top right side of the spar. The bottom two holes were positioned such that I needed a longer drill bit to clear the aft fuse. Rummaging through my box of used tools, I found a 12 inch long 1/4 inch drill bit, and a 12 inch long 3/16ths bit. This was quite lucky! I practiced with them on some scrap to ensure they worked ok, then drilled two more great holes through the spar. Not as clean as the reamer, but close enough.
With the holes drilled, I bolted on the Horizontal Stabilizer.