An LS.
All over the internet in fox chassis combos.
I'm in the middle of that decision myself. A 5.3 (5300) truck motor w/ an auto trans. Harness, and appropriate ecu/tcu's will cost you around 1500.00 for a decent 100k mile combo. You'll need to convert the intake over to a car style (ls-1/2), and change out the cam, and springs (also LS1/2) or better,.. and you'll have an engine/trans that'll be around 320hp/400tq as a starter. I'd expect at this stage your cost should be around 2k.
Add a turbo, or S/C and you'll need the appropriate fuel delivery,.. and injectors for the anticipated HP,... and you'll have a jy combo capable of with standing 700hp.
You need to have the fab skills for the swap,.... or costs will go through the roof.
You'll have to fab an exhaust as well.
A 347 will net the same N/A HP, as the LS,..and will work with your existing trans, exhaust, electronics,.... but will require heads, induction, and exhaust to support the larger displacement . If you don't have that presently,.. the cost comparison between that and an LS swap is no contest.
LS+1
Conversely,..you also have to consider that a trans gender swap is also frowned upon by all the "ford only" show/event functions that take place,.. and most ford only forums (somewhat like this one), so be advised that you'll not get alot of help from most of them for that reason. You can't take the car to a "Mustang Week" for that reason.
Additionally the swap will require pretty extensive skills in several arenas,.. and the selection of available parts that'll bolt up are just not there. So unless you possess those skills,... you may opt for the easier path using the stuff that is supposed to go in the car from the factory.
So bottom line,... you'll have to decide what you are looking for.