Not recommended
I would never recommend a VB and TC only on a trans with over 20K on it much less with over 100K. There are too many built-in leaks and issues with a stock trans. Everytime a trans shifts or the TC locks there is wear. This wear is greatly increased when towing. The clutches have to drag down the engine rpm with each shift. The Cummins torque is trying to hold the rpm, the stock trans VB shifts slow and sloppy, this makes the shifts create even more clutch wear due to the longer slippage/engagement time.
Basically with your towing history, and with an added fueling box, you have recieved very good life from your trans. I would reward it and yourself with a full rebuild and a billet input shaft. [I won't do a trans without a billet input, too much of a gamble].
With new upgraded clutches, new TC lockup clutch, new seals, tighter clearances and a faster, firmer VB engaging the clutches you will get less wear per mile than with the stock trans, and get better performance too. With a full rebuild there a many internal leakage issues taken care of.
To try to cut corners on a towing trans using a well used core is a big gamble, if you get stranded hundreds of miles from a good trans shop then you will either have a huge towing bill or be at the mercy of whoever you have to pay to repair your internals, and then still only to stock standards. You will then be out thousands and still not have an upgraded transmission.
I'd go for a full rebuild, and not gamble. When I gambled with my very young trans ['01 truck], I snapped an input shaft just from the rear axle hopping over a rough spot on the freeway, with only mild power upgrades [edge EZ] and I was not towing. It cost me $200 to flatbed my truck home where a billet input shaft and upgraded clutches were waiting on the workbench. I also lost two days work income. I gambled and lost. I don't recommend it.
Just my experiences, Greg L