Heater core clamps (1966) location?

Unless you have some whacky non-stock Chinese heater, there is no possibility of putting the clamps on the engine side of the wall.

Put the hose through the firewall in a big loop, and lay the completed heater on the car floor. Attach the hoses. I like the OEM "tower" clamps, I find them more reliable than the auto parts store spiral clamps. Install the heater, while a helper gently pulls on the heater hoses. When the heater is in place, you'll be able to cut the hoses to perfect length with correct routing. Routing matters, too, the supply hose to the heater should be on the lower fitting. The ties shown are aluminum strips, and are available, but if your car is not for show, ordinary zip ties will do.