First, welcome to the board.
Second... what is your budget? What can you afford? What sort of shape is the car this is going into?
You will want to have the cam bearings pulled and check this block for cracks under those as well, this is a big problem with the OEM blocks. You also have to see what block you actually have. If its a F4 or newer block its a roller block, if its earlier than that you will need to run different lifters and do a few other mods.
A 408 properly done with all the supporting mods is about a $8-$10k engine and that's just the engine. A good automatic behind it is another $3k or so plus all the other stuff... looking probably at a $20k build if you are doing a lot of the labor yourself. And that SHOULD be cash. If you are thinking of putting a lot of that onto credit cards. Don't. Just don't. It'll end real badly.
Not trying to scare you off, just letting you know what you are in for. If you just want a 400hp car that looks really good and you can afford a low interest car payment... I'd suggest a real good condition used 11-14 GT which you can get for $15-$20k. Few bolt ons and a tune and bang, there you go plus its got all the newer car amenities. Or... even look into a 18+ if its new car time. Those with just exhaust and tune are making 500rwhp and running 10 second 1/4 times. Complete animals in every regard.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.