A 2" PVC pipe would pass 120 GPM! (Modeled 20' long, 5' drop)
However, that size of pipe would be easily plugged with debris.
You have lots of easy options for your pond!
Maybe a 6" angled pipe through the dam with good protection against debris on the inlet side.
Maybe a 4" siphon with an air vent for automatic level control.
Do you live at the pond? If so, then your emergency spillway plan sounds like overkill to me. (And I hate not having a good emergency spillway.) However, if you live there and notice after a day or two that your outlet pipe is not functioning correctly, then you should be able to readily fix it.
If you DON'T live at the pond, even if your pond overflows, then 60 GPM is not capable of too much damage. A tiny emergency spillway of some type should enable your dam to survive with no damage until you notice and repair the main outlet.
However, you (and I) could be wrong about the amount of watershed that drains into your pond.
Have you ever observed your pond after a very large rain event?
