Un-mothballing the Six Nine Cuda BB that I built almost 35 yrs ago

I believe a good radiator shop could fix that. Maybe not the core, but the tanks for sure, as long as they are not rotted all to heck.
I had a 1960 ElCamino years ago I was restoring. Bottom tank was shot, but top tank, and side brackets were good. I took it to an old timey radiator shop, and the guy had a nice used spare lower tank on a shelf. Sold it to me along with a new core. Try the old timey places. That lower tank may fit from many other Chryslers from that era.