I decided to mount my narrowed Wassell banana gas tank using some threaded bungs welded into the backbone. I used some 1" long, 3/8"-16 threaded bungs which have an OD (outside diameter) of .75". This is far superior to just drilling your back bone and using through-bolts, which greatly weakens your frame. I marked where I wanted my tank to sit on the back bone and drilled out a .75" hole. I did this using progressively larger bits up until I got to the .75" bit needed.This photo shows the length bung I used, which in this photo is just a snug fit in the drilled hole. The bolt makes sure I didn't drop it in the frame, and allowed me to adjust the height where I wanted it to sit easier (editor: Use tophat threaded bungs to easily position bungs in your frame with no worry of them slipping through). The rear mount is slightly raised off the backbone, while the front mount will be flush mounted. I also welded up some of the extreneous holes in the frame at this time.
When welding threaded bungs, you want to take care not to overheat them so bad that they warp and distort the threads. I use anti-seize on the threads of a bolt and thread it in place before welding. If you don't use anti-seize, guess what? Your bolt may seize in the bung and make you really bummed.
Here is the finished mount, TIG welded (you can get similar results with a MIG welder), and with the filled holes ground down with a 120 grit flap disc on an electric grinder.
I now marked the center on my blank gas tank mounting tabs, center punched the mark and drilled them out to 3/8". I put my tank on the backbone, bolted the rear tab down, and very carefully marked the front mount to be centered on the backbone, and to be the correct distance so the holes lined up perfectly. I then drilled another .75" hole and tapped the bung in place. I checked the fitment several times before welding it up.
The front mount I wanted flush, so I had the bung tapped in until it was even with the tubing, and then welded it up. I had a bolt in for 3/4 of the welding, but it got in the way for the last bit so I removed it and finished up the bead. I got a little close to the threaded hole and had to chase the top 2 threads with a sharp tap after it cooled.
The tank mounted in position on the Triumph. I will end up using chrome 3/8"button head allens for a nice, clean mount, likely with rubber grommets between the gas tank tabs and back bone to provide a little bit of vibration resistance.