It sounds like your location is similar to mine, with the ravine and soil types. I have a 4' layer of red clay on top of a 4' layer of white clay, which in turn sits atop a sand seam. I initially excavated some of the sand and covered the bottom with whatever clay was handy, and used the red clay that was already in place for the top half of the pond.

It turns out, the red clay was too granular and promoted wicking, which led to a water loss of 2" per day. I ended up relining the entire pond with the higher quality white clay, which seems to have done the trick.

I suggest you do some bucket tests to determine which clay may work best. Either one may be fine, but I'd use the best, if practical.

I know absolutely nothing about installing concrete pipe. Maybe someone else will chime in. It sounds feasible to use it as an overflow. It would certainly outlast steel, and maybe even plastic. There's millions of miles of it under city streets all over the world. However, I think it would probably require some serious compaction of the underlying soil, to prevent buckling, and separation of joints. Also, I think it would require substantial support on both ends, such as, a hefty concrete apron. And, I'm sure there's probably some issues I'm not aware of.

Oh yeah, welcome to the forum, and be sure to post pictures of your project.