Jonesy, I just did my tank with new paint, trim and emblems last year. Here's what I did and it worked very well.
1. Bend them by hand to a close fit (but not perfect)
2. Cut a piece of leather just a shade larger than the emblem and used it as a temporary backing.
3. Soft mallet and with the emblem in the slots, over the leather, shaped it with the mallet until I got a perfect fit.
4. Automotive double-sided trim adhesive comes in different widths. I used the 3/4" stuff. The badge has a raised back edge. I trimmed the tape to fit exactly inside that edge. Tape went all around the badge, and almost covered the entire back surface. (I don't think it matters if the inside has some bare spots.
5. Apply to tank! Its very sticky, but with time, it apparently grips even better.