Have you check the valve springs? The fact that you made changes that should have picked the car up but didn't often points to valve springs or fuel system. Since the 60's are a little weak I would look to the valve springs first but that's just an opinion. How much seat and open pressure do you run? Also I would think that your combo would want to go through higher then 6,800 RPM so maybe a gear change to a 4.30 or 4.56 might be worth something.