Gearing - for power, speed, fuel economy?

[boyscout]

Hello, a few of you may recall that I was debating on whether or not to swap my Gas Internation 345 engine for a Diesel, or just rebuild the 345. Well I took the big plunge and decided to do the swap. I recently purchased a day-cab with a DT466, 5-speed with a 2-speed Spicer rear end. I believe this is a great combination, but it has one problem. The 2-speed Spicer rear end is geared at 5.57 and 7.30, which gives me a max speed of 56 mph at 2500 rpm. Redline is 2600, so I need to do something. I contacted the place that sold me the day-cab, and with the right amount of complaining, I managed to get them to agree to swap out the rear end. They don't have a 2-speed right now with gearing other than what I have. However, they will swap both my 5-speed and 2-speed rear end for a 7-speed spicer and a single speed rear end of my choice.

My question for you guys is, what is the proper rear end ratio to get. I need to balance power, speed, and fuel economy. I would like the ability to travel at 65 mph, get 10 mpg, and not be at readline doing so, but is this possible? Does the DT466 have the needed power?

Here is the data I have been able to accumulate; assuming a final transmission ratio (7th Gear) of 1.00. A 4.11 rear end ratio gets me 60.8 mph at 2000 rpm and 66.8 at 2200 rpm, a 4.56 rear end ratio gets me 54.8 at 2000 rpm and 60.1 at 2200 rpm. Also, what is the safe cruising rpm for a DT466?