Gearing and tire size will have an even bigger effect then engine size.
If you are staying with short tires, say 33" and under with minimal lift. A stock 5.0 will do fine. If you want it to breath a little better then swap in an Explorer intake. The Explorer/GT40/Cobra intakes have some of the best flow out there. Often so good that they are left off the magazine reviews as the aftermarket wants to sell product, and if the magazine articles say that a junkyard intake will work best for street applications, then the aftermarket will not advertise in the mags. Without advertising there is no income to the mags. So the mags are bias toward the advertisers (no real surprise).
Now if you start pushing larger tires 33" and up, then start looking at bigger engines. People love the 347, but remember that most every 347 is a built engine with aftermarket heads and good heads and lots of cubes make for a powerful engine. A 347 actually has a horrible rod ratio, the rod is angled sharply into the cylinder wall at mid-stroke. The ring package is in the wrist pin. Excessive oil consumption at 30,000 miles is common. There is a fix, a shorter rod with even worse angles to it.
351 is a much better big cube engine. It was designed to handle the larger cubic inches. It will cost less then a 347 as well and can be stroked even larger. Rod ratio is much better as the deck is taller and will fit a longer rod. Head and main bearing bolt size is larger as well. main bearings are larger. Overall it was designed to be that size. Trouble is intakes are a little harder to find. Best find is a Lightening intake.
There are other factors as well, how much hill do you deal with? If you drive on flat ground, less gear is needed. But if you must climb hills, more gear.
How fast do you want to go? Aerodynamics hit the Bronco hard. My 351/4R70W/4.88/33" combo is actually overgeared most of the time. If I only had a 5.0 HO <or> only had 4.56 <or> went to 35" tire it would be a better package overall. But the current package actually starts working good when I get to about 80MPH. I am pushing so much wind that the tire/gear/engine combo work happy (although sucking gas at alarming rates while doing so).
My current combo is also very happy if I keep it at 45 MPH, which is a good speed back when I lived in KC and drove around surface streets. Out here in the wild west, 55MPH through residentail neighborhoods is a common cruising speed with traffic.
So the answer comes down to...
Who knows. We don't have enough info. Even if we had it all there are still many opinions on what is actually right and wrong. Often 2 different things will both work with little difference.
Keep in mind that too little gear and lugging the engine will make for poor performance and poor mileage.