what wat the reason behind the FLH tank?
Better range. When we get a chance, we ride to the mountains in western NC and many of the rural gas stations there close early. That extra gallon might be the difference between an easy ride and a long push. Uphill. Both ways.
My 1993 FXR has a 111 engine in it and it is thirstier than the 1999, so the FLH tank helps a lot with it. But more gas never hurts.
The FLH tank is a very easy swap. Weld a "T" strap on the back that will line up with the bolt holes, and then weld a couple of brackets on the front and that's it, other than paint, and if the tank is the same color, then simply paint the new brackets. I put the gauges on a handlebar clamp bracket like the FXR-SP and Convertible. Easy, too, since the wires are already long enough. You can buy nice FLH tanks on eBay for between $100 and $200 if you look a little.
Bullwinkle