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.

180 Out

I'd say it's about 2" between the shock towers!  :)
-Rich