400-450 HP? Thats alot!
I have 3 words for you
5.4L SVT Blown Ford Lightning drivetrain conversion
Okay so thats more then 3 words.
Look at stuffing this into your Sport trac, easiest and most reliable way to your goal IMO.
Dont plan to have all your creature comforts when this is done, you are making alot of mods to the trac to stuff the 5.4, exhaust, cooling, etc into a sport trac but it can and has been done.
If I am not mistaken I think it was Ford SVT themselves who built a Purple Ford Ranger 2wd with the lightning conversion, looked factory and nothing to laugh at.
The 5.0L conversion is an easier fit but 450 HP is going to take forced induction and a built small block = $$$$$$$$ then you need a trans and more it gets expensive real fast and you are gambling a bit to know its all going to work together and perform
You could also look at building a bad ass 4.6L and stuffing it in there
5.0L = can fit using available parts, retain your AC key to this conversion IMO is using a 5.0L Explorer donor GT-40 engine donor from an Explorer. KNowing your ST is a Gen II ex under the hood and Ford made a 5.0L explorer shows you this is the easiest and cheapest conversion
Looking at like 215 RWHP I believe from the stock 2wd version.
4.6L can fit with more mods to the truck including exhaust, steering, brake master cyl, etc That engine is a HUGE ***** under your hood. This conversion is becoming more and more popular into rangers
5.4L has been done even with the blower, its a good drivetrain from Ford and closest to your goal numbers
research is key
I would LOVE to see a 450 HP Sport Trac hell yeah!
GOOGLE: Ford SVT ranger lightning bolt
Drool