Lay down reach under your bike with a ruler it is accessible to you. I gave you the inside pivot width of a Bonneville according to a phone call to SC asking my brother to measure one. I didn't measure a America.
I am not following why you think you have to take your bike apart to get the Speedmaster/America measurement.
Are you thinking of shortening the wheelbase by 6+ inches on a Speedmaster taking all the distance reduction from the rear? If so you really need to think about sprung weight being redistributed as it pushes on the Speedmaster front end.
