Heater core re-cored? Or such thing as a universal one?

Hey all, My 78 B body (the powertour car I'm building) has an A/C heater box, and unfortunately that heater core is discontinued in the aftermarket. The firewall and box is COMPLETELY different than a non A/C car. So my 75 parts car is useless in this aspect. . I haven't successfully found a place that 're cores. And for anything north of $150...I would just buy one of the universal under dash heaters.

Any other ideas?? Appreciate it!!! Asked on the B site and got notta