Tom, A quick check of Jim Carter offered products shows the following new replacement heater cores that are compatible with modern engine higher pressures. Not sure which type heater you have, Recirculating or Fresh Air? Both aren't cheap, but much lower cost than a higher end full system. One note on the core for the Fresh Air replacement - there may be an issue with 1953-55.1 (1st Series) heaters so it would be worth a call to Jim Carter's store to ask for details before ordering.

Recirculating round heater core

Fresh Air rectangular core


~ Dan
1951 Chevy 3 window 3100
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"My Grandpa Carl's Truck and How it Became Mine"
1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver)
US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)