For a TH400? For best all around performance, I would go the other way because top speed and highway cruising are bigger priorities
With a 4L80 the choices are wide open
Here: Mess around with a gear/mph calculator.
http://www.f-body.org/gears/
And Gear Ratios can be found here
http://www.tciauto.com/tech_info/gear_ratios.htm