I saw this post yesterday and was trying to imagine why anybody would put a Softail rear fender on their Triumph, but I've seen it done.

Now I get it,(duh). The trick is to get a fender that fits the diameter correctly. There are 2 fenders for the harleys which come on Softail customs, Dynas, and Sportsters. The bracket widths and lengths vary, but that doesn't matter. The diameter of the wheel matters much more since these fenders come on bikes with 21" wheels and 19" wheels. If you look at my fender you'll see that it doesn't exactly match the curve of the wheel, (close but not perfect). I actually grabbed the edges of the fender and stretched it wider (just a bit) since it is very narrow and barely covers the tread nevermind the entire width of the tire. This made the diameter even smaller,it's a tweaking process. I also cut a few inches off the front and rear of the fender, welded the bracket to the fender,and drilled and filled my rivets to give it that one piece look.
Feel free to PM me with any ?'s.