Bear country before, bear country today, ..........
...........and bear country tomorrow. If your trip participants are adequately prepared (mentally) to follow "bear aware" rules and procedures, I'd be no less inclined to do the float as planned. Statistically, your odds of being struck by lightning are probably greater than the odds of being a victim of a bear attack.
And as to your broken leaf spring, I expect you'll find the replacement, along with bushings, U-bolts, etc, isn't terribly expensive. I think I'd be purchasing a second set of everything. If the other end of that axle has the same leaf spring, and if it's of the same vintage (original equipment on the utility trailer), I'd expect its failure at a forseeable point in time. In fact, if you're having a shop do the replacement, I'd consider getting them to replace the other one at the same time, keeping the old unbroken spring pack as a spare. I have to wonder if the modification of the trailer and its load is in excess of the axle's and spring's rating.
But it's all good in the end. After all, you're in Montana in July. What's not to love about that?
Foy