Mod recommendations for my '84 Ford Bronco II?

Wider tires won't help with roll overs, the added traction could potentially make it easier. However if you drive reasonable speeds and avoid abrupt lane changes/collision avoidance etc you won't have any problems. Just keep in mind its not a sports car.

The 84 would have a 2.8 six cylinder, they had known issues with the cam timing gears. They are easy to change and if it has a lot of backlash in it, the gear set should be replaced.
Also it should have solid lifters and the valve lash should be kept up.

With normal maintenance 2.8 is a great engine.

You could add training wheels. I kid, I kid.
There is a theory that the reason the Bronco II were perceived to have roll over issues is that it was a fairly inexpensive high clearance vehicle and lots of young inexperienced drivers were driving these, my high school had about a dozen in the parking lot. So they would make sudden maneuvers then over correct and end up on their sides. You could look at getting new wheels with a different offset that would widen your stance (give you a little poke) that might help a little. I happen to like the plucky little SUV and have been looking for one to fix up to match/compliment the Everglades.