Engine size
 

What is the biggest engine that can be put into a '66 mustang?

well, questions like these have been answered so many times, try using the search engine. But to answer your question, a 351. But a hood scoop would probably be required for the extra aircleaner clearance, and you'd have to use shorty style heddman hedders. You could always stroke a 289 or whatever to gain cubic inches if you're really looking for size.

Without cutting, you can fit a 351W based engine. They can be stroked out to 427cid in a traditional block. The ford racing blocks can be stroked/bored out to 454. But for a street engine, you'd be better off staying with a 393 or 408. As for actual size of the engine, a cleveland is wider, and it will fit. But the cleveland has a shorter deck height (9.2 vs 9.5) and cannot be stroked out to as many cubes.

Biggest practical size is 351w. 351w is a bolt-in having every bit of power you need for the car (goes to way over 500hp). Others fit but require body surgery. The bigger the misfit, the more surgery required.