3.25 rearends 9 speed Trans Converted to 13 speed. Want to upgrade

Got a 2018 579 x15 cummins 500hp Efficiency Series . It has a 9 speed trans that was converted to 13 speed. Has 3.25 rearends. I want to upgrade to a 18 speed. What would be the best rearends to go along with it ? Best clutch as well ? I drive mainly in the Midwest ( Iowa, Nebraska, South Dakota ) then during spring and fall go out west ( Montana ) . I haul cattle so typically loads are pretty heavy . Pulling a quad axle trailer . What would be the ideal set up here?

 

That’s exactly my plan. Except I’m planning on keeping this truck for a long time. Every truck I’ve owned in the past had an 18 speed and it was nice to have. Never needed it around the Midwest really other than a few times at places I loaded. But it’s sure nice to have when your 100k plus and got a steep hill to climb and you gotta start at 0 at the bottom of it lol. Just figuring out what would the absolute best set up

 

I think what he meant was the only difference between 13 and 18 is that you can also split on the low side. Compared to a 13 you can only split the high side

 

I’d like to run 22.5 low pros. But I think I’d like to have it where when running at 65 rpm’s are around 1700. Same with 55mph. Everything I’ve read are these engines need to be run at higher rpm’s. I’ve ran mostly cats and kinda always stayed around 1500rpm. So I have a tough time trying to stay at 1700 when I’ve pretty much run 1500rpm for 10 years. Lol

 

Do you know what I’d have to upgrade to get 2050 torque ? I know someone with the x15 performance series the shop was able to do 605hp and 2050 torque. But when I talked with the shop with the efficiency series I can only do 505hp and I forgot what they said for the torque