* Type of engine: A big-block Ford (like a 460) will take significantly more modification than a smaller-block Ford (like a 302 or 351W). A Chevrolet big-block is even less likely to fit without major chassis work.
* Modifications: Are you willing to modify the engine bay (firewall, inner fenders, etc.)? Are you okay with custom headers and exhaust? A lot of "fits" become possible with enough fabrication.
* Transmission: The transmission and associated components (driveshaft, etc.) also need to be considered for fitment.
* Suspension: A very large engine might impact suspension geometry or clearance.
While a 460 (7.5L) big-block Ford *can* be made to fit a '64 Mustang, it requires extensive modification and is far from a bolt-in affair. It's often considered the upper limit for practical purposes, and even then, compromises are frequently made.
Smaller big-blocks like a 428 Cobra Jet or a 429, and even some highly modified 351 Cleveland blocks, are more common fits with significant modification. 351 Windsor engines are relatively easier fits, requiring less modification.
In short, if you're aiming for the absolute largest, it's the 460, but it will be a major undertaking. If you want something that's more realistically achievable without a complete chassis rebuild, consider a 428 or a well-modified 351. Always consult with experienced Mustang mechanics or enthusiasts before undertaking such a project.
