Gear Ratio Change? How tall can I go?

I know this is not a semi rig, but I've been trying to get an answer before I spend the money and potentially do something wrong... I have a 2003 International 4300 with 215 HP DT466 engine and 6 speed trans, 3.55:1 rear end and 19.5 low-pro tires. The truck carries a static load, about 25,000. I recently got the truck (actually put my old box on the newer chassis after the old one had an electrical fire under the dash). Problem is this truck, while it has great power, good acceleration, and I generally can climb hills in 5th and 6 gear, it hits the engine redline governor (2600 RPM) around 70 MPH. It used to be a Ryder city van, so that gear ratio makes sense, but I do more OTR driving with it and I don't want to be screaming along at redline all day, for the obvious reasons of engine wear and fuel consumption.

I found a website that does a good job of telling you the MPH speeds from common rear differentials after you input RPM, trans ratio, and tire size. From what I read my DT466 gets max torque at 1600 and max HP at 2400. I'm out east and most of the highways here are relatively flat. According to that site if I move up to a 2.73 rear end I will be turning 1900 RPM at 65 MPH, which I like a lot more than 2400. However will it be too weak that I'm down-shifting every time there is a slight uphilll grade? I don't mind using the shifter when going up hills- that's what its for after all- but I don't want to be unable to maintain speed on very slight grades. This is the question I can't find an answer to... with my weight (25k), and HP (215) is a 2.73 reasonable?

 

I wanted to post a link to the calculator and a photo of the truck but it says I have to have two posts first, so here is one...

 

http://www.angelfire.com/fl/procrastination/rear.html is the calculator I was using to determine I want a 2.73

 

Its not the speed I'm worried about, I do most of my highway driving at 65. But right now at 65 I'm turning 2400. On a motor that tops out at 2600. Seems awfully close to me. If I can get that down to 1900-2000 at 65 I'm sure I will get much improved fuel efficiency and less wear on the engine?

The only mountains I drive over are the Appalachian range, which can be steep (like on I-65 in WV) but not as long as what you've got out west. Its very rare I find myself driving west of the Mississippi, only there is a big enough story to send me there. I have 6 gears I don't mind using them, but for the average highway here that is flat, I think I could be getting better MPG if I slowed that mill down a little.