I don't know the answer to your question, and I doubt there's a web site anywhere that has the answers. But there's one thing you ought to know about the '65-'73 Mustangs, that your reference to "width, front and rear," suggest you might not know. That is, that the suspension is designed so that the springs ride in what are called "shock towers," on top of the upper control arms. These shock towers are the single most important feature of the engine compartment, because it's the distance between them that determines how large an engine you can fit.
If you find out how wide a 351 Windsor or Cleveland is -- information which IS available on the web -- you will know that the distance between the shock towers is slightly greater than that dimension, because these engines will squeeze into a '65. If you find out how wide a 390/428 is, then you will know that the shock towers are not quite that far apart, because a 390 or 428 won't fit.
Also, if you find out how long a 351 is, the distance from the firewall to the radiator support is about two times that much, minus 5-10 inches. I know this because I just swapped a camshaft for a neighbor kid and I was able to stab it in without having to raise up the engine for clearance.
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