did your car have/had a/c?
if it HAD or HAS a/c you got a core, just hook the heater core hoses to the old A/C core, put your switch to cold to get heat! and heat to get cold, that's what I'ma do when my core takes a dump.
the A/C core is meant to hold over 50 PSI I think so the coolant pressure should be just fine on it, not sure about the heat what that will do but it's meant to be more extreme than the heater core I think, it's worth a shot!!! just lengthen the hoses some, boom boom done you have a new "heater" core that's 3 times stronger than your old heater core, and it's bigger too which means you'll get MORE heat on them cold days!