My first move would be to dump that monster carb and get an 850 that will work better for you, and a converter to match. Check the cam centerline and make sure it is advanced.I am not sure where that one will run best but might need to be in as low as 100 to 102. Double check everything you have in the car, and try shifting at 6200 or less and see what happens. I think with the converter and carb, plus looking at all of what you have, you should be there without spending much more than the carb/converter money.