If it's just the idea of having an A/C system installed that bothers you (rather than an attempt to gain space within the eng compartment). You don't really need to do anything - just unplug the compressor.
But to clean up a bit you can easily:
1. remove the compressor/associated bracketing/drive belt and revert to the non-ac type bracket for the power steering pump
2. Disco the compressor hoses - suction (at the filter dryer) and discharge (at the condenser).
3. Remove the liquid line that runs from the bottom of the condenser along the inner fender and over to the filter/dryer.
4. Release and remove the filter/dryer from the evaporator outlet.
5. Pull the radiator and shroud and that will allow you to remove the condenser.
At that point the only difference you can notice will be the oversized fan housing (it contains the evap coil).
If that doesn't bother you - you're all done. There is no need to alter the fire wall when doing an A/C delete. Nor is there any reason to change the controls over to heat only. The heater will still work just as good - same with the defrost. They may work even better - I believe that the fan used in the A/C setup is of greater capacity than the heat only one.
If you are deleting to gain space - and want the evaporator housing gone - there is still no reason to alter the firewall. Several places sell A/C delete fan hosings that bridge from the fan suction throat over to the existing opening in the firewall. They also have a low profile that opens up a lot of space in that back corner - they only stick out about 3 inches from the firewall:
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1973-90 GM TRUCK A/C DELETE COVER
This AC delete cover for allows you to remove your existing air conditioning evaporator unit while maintaining the heater functions. Delete cover is manufactured from molded fiberglass with a black gelcoat and is designed to mount to the firewall where the stock AC evaporator core and case are normally located. This cover will allow more space for large valve covers and headers as well as big block engines while keeping the heater unit intact.
Fits: 1973-87 GMC and Chevy Truck 1973-90 Blazer /Jimmy 1973-90 Suburban
http://acdelete.com/pages/73-90_Truck.htm