I see, but the Explorers rob a bunch of power, more than a Mustang. A 300rwhp Mustang engine might be lucky to see 250rwhp in the Ranger/Explorer.
The exhaust is a a ton of that. Look at the total area of the pipes of a Mustang as it goes from the heads to the tail pipes. Follow it and notice the sizes of a typical well built Mustang and its exhaust. The Explorer is horrible, yet it is the same engines we are discussing, and people think a single exhaust is enough for the same engine.
The stock Explorer has collectors that each narrow to 2.125" at one point, plus the resonator has a pinch just before it at the same 2.125" size. The whole exhaust is going through that one spot, yet most Mustangs have two 2.5" pipes or larger, from the collectors to the tips.
In a perfect world you might get close to 300rwhp with GT40 heads, but you'd have to do a ton of work to the whole combination(perfect match of parts and cam, some porting), plus the exhaust.
If you can really create 300-400 engine hp, then of course bigger injectors will be needed, say 24-30's depending on what level. The computer can be tuned for anything, that's the good side.
I mentioned the TFS heads because they make more power for the given cost of the heads. If you can find them used, that may be for $750 or less. They run $1100ish or more new, and there are many choices. Just the normal port flange TW are best for most milder engines. Plus with a stock HO cam, the stock pistons should clear the valves. Check the PTV clearance before selecting rockers.
Work on the exhaust the most, it is way too small, for the stock 5.0 or any combination. It should have the equivalent area of two 2.5-3.0" pipes, front to back. Don't choke it all into one muffler and tail pipe.