Here's a oldie,but goodie: Have you visually verified, ACTUAL FULL THROTTLE mechanically? Are all 4 barrels actually opening up,when controlled by the gas pedal? It's free,usually a first step check,on something like this.
I have a mild 408 in my 68 Cuda with 3.55 gears and it runs mid 12's. Hemifish69 your Cuda should be a lot faster than mine. Something is seriously wrong. Like BarracudaBarry said you should sort out what you have first before changing things.
I agree with those that say the 830 isn't enough for maximum effort but it's plenty big to run a lot better than what it ran.
I live about 40 miles ne of Gateway. Was planning on going again on a TNT night before the season is over so I may be there the 23rd. Hope to meet up with you and outlaw