''82 I-Mark Diesel heater core is made of aluminum with plastic end tanks. Aluminum corrodes along plastic tanks, even with religious 2 yr change of coolant using Prestone 50/50 mix. Car has 256K miles (178K are mine) and runs well-32 mpg. Heater core doesn''t appear to be available anymore. Perhaps a wrecking yard but how soon would it leak? This is a lot of work to R&R. Are there any other cores that would substitute? How have others dealt with this?