Now there is an adaptor sitting in the firewall grommet and leaves me with the1/2" hose to the heater and the correct 5/8" hose to the hot water take off:
Hindsight - the adaptor actually was a saving grace, also providing a restriction in the heater circuit. When later I removed it on re-positioning the heater matrix I inadvertently also introduced a hot bypass around the main radiator and caused an engine overheat.
