[jhv48]

If you actually want a useable fifth gear, get the .80 with a 3.31 rear end.
I had the .64 fifth and a 3.46 rear with a mild 427 and the jump from fourth to fifth was about 1000 rpm. I found myself rowing back and fourth in and out of fifth on the freeway. Want to pass? Downshift. I swapped out the .64 for the .80 and now have a very useable five speed gearbox. 75mph is in my engines sweet spot. If I want to pass, I just floor it. No more downshifts.
That 1000 rpm drop is really an engine killer. If you're hot footing it and you rev out in fourth and then hit that overdrive fifth, your engine feels like it hit a wall and acceleration takes a big hit.
A lot depends on what your normal hiway speed will be. If you rarely go over 75 mph, get the .80 fifth gear and the 3.31 rear. However, if you normally cruise at 85 mph, then the .64 overdrive or a 3.09 rear end with the .80 might suit you better.