833 trans swap

I have a 68 dart with a 340 9.8 to 1 comp. ported 2.02 j heads 237 242 at .050 cam rpm intake and 750 dp. I am now running a 904 and 2400 vert and 3.55s. Thinking about a 833 overdrive and 4.10 gears. Do you think I will lose 1/4 mile performance or gain. Car runs 12.90s now.