Over the winter, I'm planning on rebuilding the heater box in my 69 roadrunner as there is something rattling around inside it when I'm driving the car. Mostly happens when the one vent cable is opened slightly so I'm guessing its a gasket or seal that is fallen off or disintegrated. Even though its not leaking, I'm strongly considering replacing the heater core as I know it will probably start leaking next year as the thing is most likely the original one and since I have it apart, I might as well do it now. Who makes the best and most durable heater cores on the market now? Or are all of them basically the same? I was thinking about checking on Rock Auto or should I avoid that? For what it is worth, the car is a non-air cond. car.