While running my very cammed 11:1 455, I had similar issues with the starter not turning the motor fast enough to start, even with a high torque starter. To get the motor to spin up to speed, I put a NC / momentary open switch in series with the coil ground lead wire.
Press the switch and turn the key. The starter will run the engine up to cranking speed. When you release the switch button, she should start right up. It's the same basic principal as a cranking retard on an ignition control module.
Place the switch below the dash on the left side of the steering column.
To test if this will work, disconnect the coil ground wire and see if the motor turns over more freely. If so, give this a try.
