You should be fine. The lead from the FW junction block to the A/C system only supplies the blower motor (via the blower relay):
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That big red wire has nothing to do with the compressor. Power for the compressor clutch is supplied on a different circuit.
Consider a heat only set-up:
Even though the fan on an A/C equipped truck is a bit larger than the fan on a heat-only truck, the blower feed on a heat-only is carried on a 2 mil wire. A 2 mill wire is the equivalent of a 14 AWG:
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Also a heat-only system is protected by a 20 amp fuse - your new harness is sized to carry 30 amps.