More likely, the higher humidity from the rain is causing ignition problems, potentially pointing to older spark plug wires that should be replaced.

 

I had this issue with the 455 in my 65 442. It had a Holley double pumper. It didn't happen overnight, but perhaps a week of sitting. I tried and checked everything mentioned above, nothing worked. I would need to pour gas down the bowl vent, then it would start right up. I replaced it with a Quadrajet, and while a little better, same issue (plugs were epoxyed). I then needed to replace the MSD ignition box as it died. Low and behold with the new MSD box issue was resolved.....car started on a dime no matter how long it sat.......well, for about 4-5 months. Then the issue returned. But based on this experience (or experiment) the whole "today's new gas evaporates" is not the issue. In this case the MSD ignitions seemed to be the issue, or the MSD distributor (had it refurbished at the same time).

I now have the stock 400 with the stock 4GC carb, and this car can sit for a month and start like it was a brand new BMW when you turn the key. I also ditched the MSD box for one of their ready to run distributors.

Again, based on all this evaporating gas is likely not the issue that causes this problem, or the weather. Damp weather can affect the distributor.

 

My car has the newer electronic distributor with coil on top of it. I could try to replace with a new one. I do not see how a spark issue gets resolved by adding fuel to the carburetor...