Looks like you are missing the heater core. I have the same heater box that you have on my 63. The previous owner of my truck stuck in the deluxe heater control panel, but didn't change the heater box

I am running a manual valve at the moment. Just have to remember to turn it on during the beginning of winter and off at the end.
I don't have any pictures at the moment, but I can get some soon for you
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Christian Carpenter
1963 C10 -
Frankentruck 283, Muncie 3 speed with overdrive
Overdrive wiring here
1963-ish truck bed trailer -
Half-Wit
1981 C10 -
Penny 305, th350 --> Soon to be 350, Saginaw 4 speed
1995 Dodge Dakota Sport
"I'll put it simple: if you're going hard enough left, you'll find yourself turning right." - Doc Hudson