I remember seeing the original '64 Mustang show car at the NY World's Fair. I alway liked the way they looked but the early ones (in stock form) were pretty "Falcon-ish" to drive (as you alluded to).
One of my old girlfriends had a 2-year old '66 notch that I used to drive. It always felt like a truck to me.
But during the time I was in college I would borrow my aunt's '61 Falcon when my TR3 or Mini was dead (often-
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I always thought that little Falcon was a very "honest" car and it seemed much lighter than the old Mustangs. Also, from that experience I started noticing Falcons. One year during a college summer job I worked with an engineer that had a '62 Falcon that was set up like the works Monte Carlo racers. It was a two door with a roll bar, disk brakes, fat steel rims, etc. He did car rallys with it.
Ever since then, I've always thought it would be neat to built a similar Falcon "Q-ship".
I much-prefer the early "rounded" Falcons to the later versions or early Mustangs.
The little six is a decent engine but no real "hot up" options due to the manifold (cast as one with the head, so you're stuck with it). Those sixes (144, 170, 200) were the first Ford "thin-cast" iron engines. Much lighter than earlier iron engines. They used the same technology to built the 221, 260 and 289 V8 engines.
I didn't exactly answer your question about Mustangs, but I just wanted to throw in a vote for early Falcons.....same basic nuts and bolts as a 'Stang, more room, and less common without getting into "orphan" cars.