I am using an under floor booster and master cylinder setup. The master is a typical dual piston. Do I need an external stop on the pedal or is the stop on the master enough?

usually, it's tough to get a nice full stroke when adapting stuff around.
Have you considered the insulation and carpet under the pedal? That may be enough to prevent the master from being bottomed out.

Pedal stop, do you mean an up stop or down stop?

With properly working brakes there won't be an issue with a down stop, the MC should limit pedal down travel and the booster will limit the up travel.

Up stop for pedal return.

Every under-the-floor brake pedal that I've seen uses the underside of the toe board for an up stop; I'd design the pedal-to-booster pushrod length to do the same.

Reason for the question, my pedal was way to high. I shortened the above mentioned rod which lowered the pedal. At the present it has no upper stop and seems to work fine. Grazie!