The 400F handles a Dyna S a little better than the 500/550 and the 250/350F bikes, but it is still a big load. If you ride mostly hiway, it can survive, but if you ride mostly city, it will have a chronic low-battery problem. Be sure to use a small headlight bulb, like a 35/35w H4 halogen, if you run the Dyna on this bike, and consider removing the front 'running lamps' (just unplug them) to help it stay charged.
If you can spring for the $$, use also the Dyna 5-ohm coils with this unit. That will cut back a little more on the loads as well. They make about the same spark voltage as the OEM coils, but with the Dyna S trigger the spark becomes very short duration above 5500 RPM, nature of that beast.