Old Sub, I might try to answer your question about two overdrive gears. Your idea to use the final overdrive gear as a 1:1 and then adjust the rear end ratio is typically the European approach. It is also the way Richmond approaches the issue on their 5 speed. Their 6 speed, however, does overdrive 6th gear.
Back to your question, double overdrive isn't really a marketting ploy. That is how the new Corvette gets almost 30 mpg and accelerates like a big block. That double overdrive makes the engine just loaf at highway speeds.
I guess it's all in how you look at it. If you're going to change the differential anyway to get a posi, or a more highway-friendly rear end ratio, then it would make sense to go the Richmond route, and have 1:1 fifth be your overdrive and take advantage of the much steeper first (compared to a Muncie for example). That could end up being the route I take.
But if you're happy with the rear end you've got, then getting overdrives would make sense to get the rpm's a little less buzzy on the highway.
A thought comes to mind. The strong 420 transmission described above, with it's wide gear separation, would do really well if you could get a Gear Vendors overdrive unit on it. It would turn the 4 speed into an 8 speed with the 0.78 OD. Try that with a close ratio Muncie and every overdriven gear is equal to the next gear anyway. But on the 420, it might actually give you 8 real gears to choose from. Hmmmm.
