I ran into a little different but similar problem. I installed an aluminum radiator to cool down my big block and was running a 14 pound cap with a stock heater core which is rated for 7 PSI. I just recently found an aluminum heater core that is rated for 25 PSI so i will be doing a pre-emptive strike and remove the old one before i have a problem. If it is a deluxe heater I think the core comes out on the engine compartment side.
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