I would bet they will fit at the engine; but the 2014+ Valk muffler sections are considerably shorter then the stock GL1800 and F6B muffler sections.
Which means you would have to fab some rear mounts. And unless they were to be hidden by Corbin or custom saddlebags, fabbing the muffler mounts could be difficult to implement and remain … non-aesthetically challenged...

If you are mechanically inclined, you might pick up a GL1800 exhaust pipe/muffler for one side on ebay, and see what you can do. I think it's possible; but the aftermarket pipe vendors are apparently to lazy to figure it out; prolly not enough potential sales to devote the time.