coleslaw - you could actually leave the rest of your system as is and just put adapters on the heater core or if you use silicone tubing stretch it over the 1/2" barbs.
Remember too that by going from 3/8" tubing to the larger diameter heater core you would be increasing the time that the water spent in the radiator, giving it more time to cool

If you end up going that way, remember that you can get good heater cores at your local Auto Zone, NAPA, etc. for under $30.