If you upgrade your trans to an Allison World you will need an axle ratio of 3.90-4.10 depending on tire size. 4.10's will get you 66 mph@1375 with 24.5 rubber with the World .64 OD or 3.90's with 22.5's.
What exactly is their excuse? I got the same runaround with Cummins when I jacked my N14 up to 525 hp/1850 torque. Said my trans was only rated for 1650 so they wouldn't do it. Last time I checked I OWN the engine AND the transmission and everything bolted to them. I took it to my local International dealer and they were more than happy to take my money. See if you can find an MCI or Prevost shop or a truck dealer like Freightliner or Western Star that is certified Allison and see if they will do it.