There are a lot of good boxes for producing max power, the problem is that some are not particularly good at towing. High EGTs are the problem and towing heavy means you will be under load for extended periods.
I made one 4,000 mile trip stock, pulling my 15,000# triple axle. On the next trip, I had installed the Edge EZ and it made a world of difference on the hills. I found that Level 3 produced the torque I needed and higher levels only caused the EGTs to go up dramatically and were unusable for towing.
Whatever you decide on, please monitor your EGTs. I found that on the steepest grades I would need to downshift to 5th. and watch my EGTs carefully. You will be surprised at how infrequently downshifting will be required, especially if you can stay in the 2,000 - 2,200 RPM sweet spot. Like you, I have 3. 73 gears and that means my sweet spot is 70+ MPH. With your 17,000 pounds, even with trying a lot of tow angles and weight distributions, keeping the the sweet spot may not be comfortable or safe cruising speeds. On long stretches out west, with limited traffic it may be Ok, otherwise better safe than sorry.
If you are going to do more towing than empty, 4. 10 would be better for your weight.