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I would take any new core and test for leaks, then I would have the tubes reinforced with additional solder, because the connection point is where they typically begin to leak, especially after someone starts yanking on the heater hoses to remove them.
I use a razor blade to cut off hoses, to avoid stressing them. don't forget to reinstall the brace that holds the tubes in place. ... protecting them from Hose yankers !
you do not want to rip the dash area apart, unless you absolutely have to. good luck.