. . There were two methods. The mounting details depend on the year of your TM.
. . Horizontal gas tanks have a lip or folded edge, maybe an inch wide and 8" long, along the bottom of the rear mounting base. You can see it in the lower right corner of the attached pic. When TM first started using horizontal tanks, you would lay the tank in place, with the lip sitting on a metal shelf near the rear of the TM's A-frame. The lip and the shelf each had two holes, and when the tank was properly positioned, the pairs of holes overlapped. You would drop a bolt through each pair of holes, and screw a wing nut onto the bolt from below. Because the bolts were pretty far under the TM, and hard to access, mounting and dismounting a tank was a PITA. So after a bit, TM improved it.
. . In the new method, the TM put a pre-formed horizontal slot at the original location, and the rear lip of the tank was simply slid into the slot - no bolts needed. This held the bottom of the tank in place. The top of the tank (near the gauges) also has a lip (visible in the lower left of the pic) which was set down on a shelf nearer the hitch, and held down with a short unthreaded pin, which was easier to access and easier to insert/remove than the bolts in the original mounting method.
. . Both methods are simpler to do than to describe in words. Neither method required any mods to the tanks - they worked just as they came out of the box.
If you have a 2006 TM, you probably have the first method.