did 65 darts have overheating issues?

I can't think of any Mopar, A-body or otherwise that had an overheating problem in stock form. In fact, the stock cooling systems (22" radiator) will be just fine with a substantial increase in power.

The grille area on your 65 is more than the radiator area, so you're fine. Opening up the valance will significantly increase the chances your radiator gets destroyed by a rock.