Heater core source. 58 Apache std not deluxe heater.

[Rm58]

Gathering parts for my first build, 58 Apache on 98 s10 blazer 4x4 frame with 95 TBI 5.7 stock engine and Trany. I have most of the stuff but seems like the heater core is a little hard to find for the standard heater. As you can see the truck is custom but I like the simplicity of the heater and am going to reuse it but the core is bad. Anyone found a source for one that fits the box. I have tried getting mine re cored and the first place said no but haven't given up on that. Love reading about everyone's projects appreciate the guidance.