Cat 475 & 550 both 1850 torque?

So I've been thinking of buying my first truck and been looking at '99 Pete with a 475 Cat and 18spd. I forgot to check but I'm assuming with that year of truck it's a 2WS, is that correct?. Now naturally my first thought was what would be involved in getting 550 out of the motor assuming the driveline can handle it. Truck has heavy 40 rears so that will be ok, but I didn't ask what the clutch was rated at. I then read that both the 475 and 550 are rated at 1850 torque so theoretically then, the 475 clutch should handle 550. So that got me thinking, what's the deal with that, and how is that possible? If they are both 1850 torque what kind of performance increase could I expect going from 475 to 550? Pulling hills? Fuel economy? I'll likely mostly be running around 90,000 lbs gross, however I will be doing some B-train work at around 140k as well.

If I decided to go ahead with it, is there anything other than an ECM reflash needed to get to 550? Hardware? I'm sure this has been asked before, and I apologize, but my searches have been unfruitful. Thanks fellas

 

I guess what I'm most wondering is if they are really both 1850 torque what I would potentially be noticing going to 550 while driving it.