Hey Tim,
You can get the core from the brass works co. It is expensive if you want to go that route. I got creative with mine like Red 58 suggested and I did some research after Red gave me the exact dimensions for the core. I had nothing to start from but some configuration someone had made from some copper tubing. The only core I could find over the counter was for a 71 Ford Pinto with air. The dimensions are close but you have to do some fabrication to attach the core to the heater box with a few pop rivets. The core is all aluminum. The inlet and outlet on the core with air is at 3 and 9 on the clock. The core without air has the inlet and outlet at 12 and 6 on the clock. It will take some tinkering and measuring but it can be make to work. The Pinto core cost me around $26.00 and I used some scrap sheet metal to make the brackets. I'm not worried about being original all I wanted was to be warm while driving. It's like Red said . Don't expect to feel like you are in the living room with the old wood stove roaring but it will be adequate to enjoy the drive in your long sleeves.
Oh and by the way my truck is a 58 apache with the 235 and overdrive. Might not look like much to others but it's mine and it takes me anywhere I want to go whenever without hesitation.