3,400 sounds a bit high for a 68 coupe to me. Then again when you add a big motor, OD trans, and a 9" it adds up quickly. What does 16 gallons of gasoline weigh? IIRC it is about 8 pounds per gallon for water but gas is lighter.
I have 300 rwhp and 400 ft lbs of torque at the rear wheels in a 351W that weighs 3180 lbs. I just got a new Edge Racing torque converter that stalls between 2800-3000, which is pretty close to my peak torque. As is, it is a 12 second something car. How much nitrous do I have to add to get to 11.99?