Don't know if these will help but, I took them during a big block heater core conversion.
67 Camaro 350 no A/C
This is the inside firewall directly above the trans tunnel:
This is inside firewall with heater box removed:
Sorry no pics of it all assembled. I do remember having a hard time getting the defroster duct back in place because of the center console, should've just removed that console but I opted to struggle against it. All in all it worked out well.