Some have said that the "Sabre" backrest sits too high on the bike so what I did was to re-drill the mounting holes in the backrest bar and lower the backrest about 2". You will need a #9 drill and a M6X1.0 tap. The existing upper mounting hole on the bar are now the lower mounting holes. I drilled + taped two new upper mounting holes (you can also change the angel of the backrest if you want). I cut off the excess bar about a 1" below the lower holes. This set up works great with the mustang wide vintage seat. I have attached before and after pics including one with the mustang seat.
Before
After (stock)
After (Mustang)
