What gear ratio or retrieve per turn is better suited for the following:

6" shad hollow belly with a 3/4 oz open jig head?

I throw my hollow bellies on a 7 foot M/H fast with a Lew's tournament mg in 6:4:1 ratio

I prefer the 6.4:1 as well.

I'd stick with 6 or higher. Not all 6 gear reels are created equal. Make sure to look at the inches per turn or IPT. Some 6 gears will put out 26 IPT and some put out closer to 29 IPT.

I'd stick with a reel that puts out no less than 26-27. You could even use a burner reel as the bait will not have that much resistance coming through the water.