Dennis,
I have a 70 with A/C. The heater core is located inside the distribution box found under the dash on the passenger side. There are 4 nuts in each corner on the fire wall around the A/C blower. Remove those nuts and heater hoses.
Also remove the distribution duct work from this unit and also unplug the group of vacumm hoses that come from the a/c dash control. Unplug the connector plug with all the hoses attached (don't unplug the individual ones!)
You can now pull the unit forward into the passenger compartment. There are 4-6 screws that retain the cover of the heater core (this cover was previously agaist the firewall) remove the cover.
Unfasten the core and remove. Be sure to seal the cover up agin using plumbers putty or a seal from any of the restoration catalogs.
Good Luck!
--Andy--