How much gain with lower gears?

The original question will be very difficult to predict without knowing the torque curve of the engine. In very general terms, the reason a gear change makes some cars go quicker and others run the same ET is because of the torque curve of the engine. Better ET's come from maximizing the amount of time the car spends in the RPM range with the highest torque. If your car makes lots of torque down low, and falls off as RPM gets over 5 grand, the car will likely perform better with a higher street type gear. If the engine makes good torque higher up, a lower gear will improve ET. You said it was a "mild" 408. That might mean it's got a good street cam that works better at lower RPM. If that's the case, a lower gear might do more harm than good.
I'd spend a little money on a couple of dyno pulls before making a decision on a gear ratio.

Garry