You can remove all the duct work shown in the image below - except part no. 14 (defroster feed):
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Also the control panel for A/C is vacuum operated. It needs to come out - along with all the associated vacuum tubing. On the engine side you can lose the vacuum reserve chamber - over next to the brake master cylinder.
Replace the A/C control panel and hoses with a cable operated unit - like this:
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The fan speed wiring is the same - you'll use the same harness connector. The A/C system has an extra wire that goes to the temperature selector - you can snip that. The hot wire for the compressor; comes off the fusible link gets eliminated. And I think, IIRC, that the connector for the blower resistor is different on a straight heat system.