recore or new radiator? 70 dart slant/6

I assume it's a stock slant 6 radiator? 19" or whatever that size is? If so, unless you're trying to go full original restoration, I wouldn't re-core it. Those are wimpy radiators.
If you want to keep the stock-ish look get an early 70s 22" Mopar radiator and re-core it. It will bolt right in.
If you don't care about the look just get an aluminum one and call it a day