For those considering the ProTac backrest, I can tell you it's not quite a direct bolt-on for the Speedmaster. Maybe the single/pillion setup on the America is different, but the gunfighter style on the SM is slightly too wide. I really like the backrest, and the price is right, but if you decide to go this route, you'll need to accommodate about one inch of interference between the seat and the two supports for the backrest.
This is what I did. There may be a way to modify the bracket, but I chose to modify the seat instead. I marked the side of the seat where the backrest had to go, then removed the seat. Below the marked area, I removed two or three staples holding the cover to the seat pan. I then used a rotary tool to grind off a half-moon shaped notch on both sides of the seat pan. I made it about half inch deep and about an inch long. I tried to smooth the edges so it wouldn't leave a sharp edge that might damage the cover. Then I just pulled the cover back into place and replaced the staples. Now the backrest slides right into place, and when it's removed the seat looks stock. For $125 and about an hour's effort, it works great.
