Maybe you can try the 3.07s before you switch it. First gear will be like a slingshot. In fact, my last car had 3.08s in it too. A lot of guys run 3.07/3.08 with the FEs because the have the torque to pull it through first gear and lower gears tend to shred the rear tires anyhow. Part of it also depends on how heavy your cars final weight will be?. A 2000lb Cobra is much different than a 27000lb Cobra. Many of us have found out our cars are much heavier than we first thought. Those 305s are pretty tall too. If you search around on CC, you'll find bits of discussion about many big block replica cars going back to taller gears now days. Hmmmm is right.
Does your automatic have an overdrive gear?