Hello Larry.
Some years ago i had a problem on my 47 windsor with the same symptoms, after checking so many things
i found the problem! Where the fuel enter the carburator,seat of the float needle part no.690676,there is a rubber seal where the needle close,that hole in the rubber was too little and i decided to drill to open it and it was just that. There wasn´t enought fuel when the car reachs 50/60 MPH. Check this on the carburator its easy to do.
From Portugal
Paulo