Head gasket issue.

Combustion gasses are being forced into the cooling system forcing the coolant out.

When the heat was good, that was when your coolant was full, and as it started to push the coolant out, it overheated, and the heater core was not getting coolant flow from the hot engine(because there was no coolant flowing) and stopped blowing hot.

Head gaskets, and or head rebuild is in your future.

87 Raider 4D56td v5MT1
31's..Basically Stock