I'd stay away from the import motors. You can get more power with less money using something more common in the mustangs.
If i remember right, the first generation mustangs didn't have a big block in them due to the lack of space between the fenderwells. I think 1967 was the first year for the big block in mustangs/cougars. That rules out the 427. 428, and 429. 427s and 428s showed up in later models so you could probably make one fit with the right combination of parts. I'd also stay away from the 429. Not a lot of aftermarket support since these were mainly big car and truck engines.
Personally, I think a 302 stroked out to 347 or a 351, with some work done or stroked, would be a better combination. The 302 seems to be a very dependable engine with lots of horsepower potential. Use the right combination and you can have a screaming small block with an easy 400+ horsepower. Plus, there are endless possibilities for the 302s and 351s due to the popularity of the fox-body mustangs.
Put an all-alloy fuel injected twin turboed 427 cammer in it.
Who gives a stuff how hard and expensive it may or may not be when you're playing with well over 1000 horsepower at the wheels?
As far as import motors, there is enough room in that engine bay to fit almost anything.
I personally would go for a 3UZ-FE (4.3 litre V8 that is powerful and smooth as silk) or a 1GZ (5 litre V12 that is about as powerful as the 3UZ but torqueier and even smoother - it is much rarer though) from Toyota, or a VQ35DE (3.5 litre V6 from the 350Z) or VH45DE (4.5 litre V8, about the same as the 3UZ) from Nissan.
429 is the same block as a 460. The 427 is a FE style engine. Both will require cutting shock towers and changing front springs. Bolt in without modifying choises in V-8's are 260,289,302,351W and 351C.