I have read a thread on this subject in one of the other Hot Rod forums I belong to, but I'm sorry I cannot recall what the fix was if any..... The thread in question was written by a driver of an old ford coupe... You might try searching the threads on the NTBA site and also Old Skool Rodz..... Seems like the thread was atleast two years ago....
If I recall the answer was to tilt out the windshield slightly, If you can do that with your windshield, you might try it. Or maybe try some of the 3-m window film used on south facing windows of houses....I know this is a problem, so good luck, I hope you find a cure..... "BH"