I meant to post this a while ago, but in case useful Pro Alloy in Suffolk have a pretty good 240z replacement alloy heater matrix available.
Datsun 240z Heater Matrix
With the original Datsun parts now almost impossible to find in working condition, we have developed this replacement part with all the reliability of a modern high quality radiator. Our version is approx half the mass of the OE part and is installed in the same location with no need to modify...
www.proalloy.co.uk