Sounds like it would be really loud! But seriously, you don’t need to worry about balance etc if you’re doing a custom frame. Just put things where you need them to balance it out. You’d basically be building a truggy and putting the 4Runner body on it. Have you measured the engine for clearances under the T4R hood? 7.3s are, as you know, BIG engines.
Do you have the 84 already? Is it in good shape? If you’re going to do custom everything, why not sell the solid axle truck to someone who will keep it closer to stock, and hack up a cheap ifs rig?
Personally, I think it’s a waste of time type project, but I don’t have any time to waste, so what do I know?