I know the head pipes are different between the RD400 and the Daytona. Do the mufflers interchange though?
Not without work. The rear mounts are not the same.
I have had an RD400 with a Daytona motor in it, and used the complete RD400 exhaust so it bolted to the frame. If I remember right I think I had to elongate the holes for the cylinder studs in the head pipe flanges. I think the Daytona motor has wider stud spacing. I have pics somewhere.