If you decide to replace your heater core, contact heatercraft. The new heater assemblies are much much better designed, they now have what appears to be a bent copper tube that has fins on it, not the fragile aluminum heater core.
Heater craft can sell you just the box and the core, which is a much better deal than buying a completely new unit. Your old blower will bolt on to the new box. Did this last year, and it works great.