You can't drop that thing onto a Mustang chassis.. the Mustang has no frame. Like mentioned, it's a uni-body car. The frame is built into the body.
Dude, I just gave up on you when you said you would rather have a Civic or a 1950 Ford with a flat-head.
Also, you mentioned you were going to drop the Ford onto the chassis and put an intake on it? That's it.. dude, I'm seriously speechless. Not to mention, the wheelbase of a Mustang is most likely no where near the same as the 50 Ford you mention.. it's not like an Xmod where you can just throw in a spacer and make it work.
You have absolutely no idea what you are talking about, what you are saying.. you're just talking out of your butt hoping we will go "OMG LOLOL GREAT IDEA MAN DO IT HAHA WE THINK THATS GREAT!"
Little did you know.. we know what we're talking about.
My suggestion? Take some time and do some research, and get to know what you are doing, what you are talking about.. and realize that a 1950 Ford with a flathead is way cooler and much more rare than any Civic you will ever find. Civics are a dime a dozen, you see them riced out everywhere. How often do you see a 1950 Ford cruising down the road? Point made..