1989 Daytona ES Manifold crack?

Hello :) Car overheated. (Noticed heater wasn't working.) Radiator fine until then - fluid loss. Replaced thermostat. Heater working fine now. Still overheated. Replaced bypass hose below the thermostat to intake manifold and replaced hose coming from intake manifold to heater core. Although not overheating now there is still fluid loss in radiator (roughly 4" top off). I do check and fill the radiator before every drive. Any ideas? Need to replace entire manifold? And if so, where would I find one? (Been searching internet to no avail.) And if not the manifold then ...? Any suggestions will be greatly appreciated. Thank you. :) PS: No oil in the water.