The problem is with your output from your sending units. GM and Ford/Chrysler are totally different. The output from pre 1989 ford/Chrysler's is 73 ohms Empty and 8-12 ohms Full. GMs output for pre 1965 is 0 ohms Empty and 30 ohms Full. 1965 an up GMs are 0 ohms Empty and 90 ohms Full. You'll need an early GM sending unit. If you junk yard one make sure the tank is as close to the same depth as your Bronco tank or you won't get an accurate reading either. You could also check with some of the GM restoration outfits for a universal 30 ohm sender.