I used to run a 468 with the steel 990 heads that I had reworked some and the flow numbers were similar to yours. The car was 2700lbs and ran 9.50 with tunnel ram and dual 750's. I changed to a set of Dart 320's and was disappointed to only pick up about 1/10th, but the Dart heads were better on the 468 and you have a 481. I did additional work on the Dart 320's and was able to get the 468 to run a best of 9.19. So based on that, I would think that with nitrous you would need a bigger head. It should still run good, but could possibly run better.
The transmission sounds good. It has things in it that it needs to live. If you are running only 1/8th mile, you could change the gear on up to help move the car quicker. With the 7200rpm, 29.5 tall tire, and figure max of 125mph in the 1/8 (allowing for the nitrous), you could go up to about 5.00. So on the conservative side, I would go with the 4.86. It will have more of a tendency to spin with the steeper gear, but it sounds like you are doing everything right on the car.
Just some thoughts,
Good luck,
Bill M