Going From A Pre-Emission 3406e 550hp to an EPA ISX 475hp (need engine/ ratio advice)

Those figures are about right I'd say. According to my gps, the speedometer error is just under 1 mph.

 

If the recon engine is an 07 or before , I would think your only issue , outside of gearing , would be doing an EGR delete. ISX is a good engine , except for the EGR.
As far as your gearing , I'm not sure what rebuilding your 3.36s would cost , but here's a thought. Run it the way it is , for awhile , until you're ready for new drives. Then find a good salvage yard , trade out your 24.5s for 22.5s , buy your new tires , you're set. Wish you'd have posted this 6 months ago , cause I was wanting to swap from 22.5 , to 24.5 . I did , and that's how. Cost me $500.00 for the rims , and I needed new drives anyway. Lot cheaper than swapping rears.
Good Luck !

 

As to the weight , you should be able to find out the dry weight pretty easily . If its not on the door tag , get the serial # , call a KW dealer , and ask the parts man . He should be able to look it up.
From that , add the weight of oil , coolant , and fuel, and you're set.
Or , just drive it to a scale , and weigh it. Then figure 7 lbs. a gallon for the fuel.
If you are really that obsessed with getting a tag axel setup , for the fuel savings , you may be better off just finding a truck already set up that way.
IMHO , you won't be happy with that setup. I ran an 07 Pete , 550 Acert , that locked , I could , and did , use it a bit , in Georgia. We didn't have snow to deal with , we had red clay mud. When its wet , it's worse than snow , almost like ice. If it was hard-packed , it was like ice. All 4 screws would be spinning , and , while I was always able to eventually get going , I had some close calls.
If you're running the Midwest , I'd be thinking long and hard about the inconvience of getting stuck , the cost of a tow , as against the Possible benefit of a very small gain in fuel mileage. I ran the Midwest for a lot of years , and no way I'd want a single screw setup.
But, to each his own. Good Luck , whatever you decide !