@DemonMudder This thread definitely sees some action! Nice work, looks like you're the type who knows what he wants and makes a plan! That said, you're definitely on the right track, not ditching the SOHC. People can bash it all they like, but the truth is that the Cologne V6 is one of Ford's better designs, in terms of reliability. You'll know the timing chain noise once you hear it. Clacky sound. Whatever noise yours is making, it's not that.
Gears. Everybody wants to change gears when they up the tire size. Good choice. Keep the SOHC and the 5R55E as long as you're not towing anything; that's a recipe for long life, but you won't win any races in it. These trucks aren't designed to go fast, and they don't. Don't even try. The beauty of these things is in the torque control, and for that, you'll go very slowly. Use the shortest gear ratio you can afford; I run super fat 33x12.5's on 15x10's with 1.5" spacers, and I usually get around 15 MPG on 4.10 gears. Since gas isn't an issue for me, I've got 5.13's sitting around waiting to go in, but I have a separate daily.
With this setup on 4.10's I dragged a dead 5,000-pound Dodge Durango around town without even feeling the weight behind my '99 Sport. Torque over speed every day, baby.
"Sport" is another factor: I respect that your 4-door is a great deal heavier than my 2-door, and that should be considered when deciding gears. I'd still recommend the 4.10's since your tires are a great deal lighter than mine. You could probably find a Ranger or Explorer with the 4.10's and pull them yourself at a junk yard, I think the axle code is D but don't quote me on that, and don't assume that the door sticker means factory components are on the junk yard trucks. Always count the teeth. If you want 4.56's or 5.13's, go for it, but you probably want taller since you highway a lot. I think 4.10's is the best bang for your buck, and easiest to come by, easiest solution to all of your problems, and by far the cheapest.
That business of your old junker V6 running better than your family's V8's is no coincidence. It's designed to work perfectly for decades if maintained the way it's designed. They messed with theirs, and wrecked them. Mess with yours, and you'll wreck it. The only way to change it correctly is to change all of it at the same time, and then it ain't an Explorer, so for the price, just buy a separate truck for that. Keep the Ex an Ex, or build some awesome junk like
@410Fortune does.