Assuming you have the original Rochester carb:

Q-Jets as a general rule have no personality. They either work or they don't. Also, the engine should run BETTER, not worse, under damp cooler conditions (more oxygen).

Possibilities:

Partially clogged fuel filter
Partially clogged fuel sock in tank
Ignition
Icing (rare with a large 8, but it does occur, particularly if headers and/or aftermarket aircleaner)

If the "dry gas" (alcohol) is helping, I would try a tank of gasoline from a different source. Here is Missouri the humidity often exceeds the temperature, and I have not ever needed (in fact avoid) the alcohol in over 40 years of driving.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
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