445 Mp8 Mdrive and 3.08 rears

Anthems can be customized to come with an Allison or Eaton trans. Mdrive is the base choice.

 

It depends on which 445HP engine and program you will have. The 445 turbo compound (MP8HE SE.) should pull well with the 2.64 as it puts all of its torque from 1000 RPM and has 1860lbft of torque, but other specs like the MP8 MaxiCruise/EconoCruise/Maxidyne will require higher value rear ends because they just put the torque from 1200 RPM to 1400 RPM and can have anything between 1560 to 1860 lbft of torque, depending on the program you select. Above 1400 RPM all 445's should pull the same.

Without knowing the rest of your specs, if you want full pulling capacity without excessive fuel consumption, you should spec the truck to cruise between 1300 and 1400 RPM, anything above that will waste fuel; And if you are too concerned about pulling capacity, you may want to get the 505Hp with 1860lbft engine instead of the 445Hp. Also, if you want, you can spec the truck with any Eaton Auto or Manual and even with a Mack Maxitorque manual, but in the field of automated transmissions, the Mdrive should be superior to the Eaton.

*Note: Without Knowing which ISX 450 you had, is difficult to know why you feel it to be so guttless, as it could be spec with 1450 to 1650 lbft of torque (Or even up to 1750 lbft in the Smart torque versions.)

 

If it was me. I'd look at the speccs that Joel Morrow of Ploger transportation. The Turbo Compounding engine. Along with a super Overdrive setup they are running. 13-14 speed transmission with crawler gears. You can run high rear axle ratios like 2.05 with the 14 speed. You can run highway speeds in direct and overdrive. But you get the good pulling with the crawler transmission. It's a win-win.
I have a VNL740 with the Volvo downsped version. D13 455hp TC, 2.47 rears and 12spd I-shift. The setup is pretty legit. Pulling tanker 79k all the time. It pulls way down low. 1,850 at 800rpm up to 1,400rpm. I imagine the 13-14 spd setup is much better.
I'd get on Volvo truck masters on Facebook. Joel Morrow posts on there all the time. He posts a lot there.