Can a 400 engine fit a 1964 mustang?

Short answer, no. Long answer, yes, but not without major modifications, and would you really want to? For one thing, the front suspension of the 1964 1/2 Mustang is not nearly as strong as the '65 and later, so it would be inadequate for the added weight. The 400 engine is fairly tall, and I don't think it would fit under the hood. A better choice of engine would be a 351, which I know first hand can be stuffed into a '67 Mustang, but I still wouldn't do this to a '64 1/2, due to its structural deficencies.