I hate to come out and say this but something is terribly wrong with this combo and the convertor is probably a BIG part of it. You should be flying at that track especially this time of year. I sold my first Duster to a Friend around 35 years ago (3100 plus pounds) and he was into working on dirt track cars and wanted to try drag racing. He wanted a mild combo to make some passes so we took a stock 340 block with stock 10.5-1 cast pistons, stock rods, and crank, added a 484 purple shaft hrdralic cam, portasonic intake, 340 heads with a clean-up polish and 2.02 vales and valve job. He then bought a new 750 carb and installed it out of the box. The car had super stock leaf springs, 4.56 gears, and a 4200 stall turbo Action convertor. In the heat of Summer in PA at the old Keystone Raceway Park he ran a 12.20 on his second pass in the car. I would take a real good look at your 3500 stall convertor and possibly try a new higher stall one.