I would think that any place that analyzes oil should be able to do trans fluid as well, it's still oil after all, just with some hydraulic fluid mixed in as well.
My T56 is the 95+ model with the 2.66 1st. I believe that is the most common gear spread for T56s, not sure on that one though.
As that shows, the NV4500 has that short 1st gear which would be very helpful in getting a very heavy load moving, and as you pointed out, 5th is nearly identical.
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anyways, from a purely mathematical perspective..... the T56 gear spacing is VERY nice, has fairly consistent ratio changes, which would certainly help keep the engine in its powerband at any time. the NV4500: not so much. shifting at 3000 will land you anywhere from under 1500 to 2200 in the next gear. some gears, you would need to wind it out a lot further than others to keep the engine from dropping to too low of a speed in the next gear."
The same closely spaced gears that are enjoyable to just drive/hot rod around in are also helpful when towing because as you stated, it will help keep the engine in the fat spot of the powerband each and every time you shift.
The NV and T56 are both great gearboxes. Just depends on what you want to do the most. Do you want mainly fun cruising from a brick-shithouse-tough trans that shifts like a sports car and can still do some occasional towing. OR, do you want a true truck trans with that granny 1st gear to really get the load moving. The NV is a wonderful box and certainly has it's place, but it will never have the shift feel and shift throws of a T56 (obviously). Don't anyone misunderstand me, the NV is a great box.
　
There are/have been/will be some companies offering 4wd T56, not sure if it's available yet? Anyways I have thought much about a 4wd T56 myself, and I know there are people out there who have done it; I believe it usually involves running a divorced T case but check this out
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