Not difficult at all if you're a master fabricator, a clever driveway mechanic, or you find a ready-to-weld kit specifically for Early Broncos. Other than that, I'd have to say (never having actually done one myself) that it's a crap-ton of work for minimal benefit.
Never seen one of those kits myself, but with all the custom metal fab shops across the country and the popularity of EB's, it wouldn't surprise me to hear someone is making an EB specific kit.
You need to work out the geometry.
You need to work out the materials.
You need to work out the layout, specifically for whatever shock length and size you choose.
You need to initially estimate the weight and bias of your individual Bronco to get the correct spring combinations.
You then need to mock it up to make sure that absolutely nothing is going to be in the way of anything else, including your new, wider Coyote engine.
Then you need to re-do anything that doesn't play nice together.
Then (it seems) you need to buy a second, and sometimes third set of springs, when the ones you estimated and that other race shock shops told you would be perfect, turn out to not at all satisfy your needs.
Seen and heard about all of the above many times. Not just with Broncos.
Other than that, it's pretty easy and well documented.
;D%)
So my usual comment is that, since it's a street driven Bronco, the only thing you gain with coilovers and/or reservoir shocks is the big boost in the cool-factor category. None of that stuff is necessary for improving the ride or handling of a street driven EB. Unless you're just into dialing in the perfection of your ride to the Nth degree and have the knowhow to do it.
All that said, there are plenty of people around that do use coilover setups and love them. Not sure if they feel the work was worth the result, but depending on what their goals were, there are likely going to be lots of encouragement for you to go for it.
So hopefully they'll chime in with the details of their builds so you can see if the end result is near what you want yours to be, and if they think it's worth it.
Good luck.
Paul