Alfonso,
Superformance has dealers in the UK, and in Europe (in the Netherlands).
Here is a listing of all the dealers:
Superformance | Dealers
From what I have read here, it seems like the UK dealer, and Dennis Olthoff are much better than the other dealers, in that they know much more and they are modifying the cars before delivery to the customer to improve them. I've met Dennis and can say for sure that when (not if) I get my Superformance, he will be the person I go to.
His shop is in North Carolina.
You should check out the Superformance portion of this website for much more information.
If you are going to be putting an 'old' engine in your car, I would hope that you would put a 427 side-oiler, or a 428, as they are mostly the same (the 427 is a better engine, but you can still get a great quality motor with a 428 block), and the 427 is what the car originally came with, so it is "right".
Where are you located in Spain? I travel to the Cadiz area frequently....