Fastback ´67+crate engine = ?

Hi everyone!

Hopefully someone could help me out here, I don´t know much about engines and all that mechanical stuff. This is just a theoretical question: what is the biggest, most badass engine you could fit into an old Fastback?
I´m currently building a scale model of a Mustang Fastback ´67 and it will need some more power than the stock 289 c.i engine. I´m thinking of putting in a Ford Racing crate engine into it (something like this, but maybe they´re too big, I don´t know (http://www.fordracingparts.com/crateengine/bigblock.asp) ), it will be supercharged and nitrous-fed. The actual reason I´m asking is that I like to keep things realistic on my models. It´s going to be a streetcar, not a funny car or something that crazy, it will appear stock so compressors or blowers are not an option.
Another mega-n00b question, what is the actual diference between a "crate" engine and a factory engine?

Thanks in advance!

B-E-A-S-T:
http://www.fordracingparts.com/images/part/full/M6007D514.jpg

Someone makes a 351w based 427 stroker that is evil. In my estimation, it's probably the meanest motor you can fit in a vintage Mustang without cutting out the shock towers. For a scale model though, it would just look like a run of the mill 351w. Find a picture of a 428CJ motor. That was the biggest motor that Ford stuffed in a fastback.