Not that my comments below are the right answer for you pond location, but, for an example, my pond was originally built in the late 60's/early 70's and the top of the dam (with 2 foot of freeboard) was about 6 feet wide. I don't really know if it had a legitimate core.

I had the pond renovated a few years ago and the muck was pushed to the back side of the dam and dressed out. It is now basically 20 feet across at the top and so much nicer to mow and hang out on.

The real concern is if you think your pond will overflow the dam. If so, the wider the better. My emergency overflow area is more like 40 feet across. A narrow dam will wash out much faster if/when your through pipe is overtaken and the water flows over the dam.

Just food for thought really,

Fish on!,
Noel