FP, glad you like the new avatar, that's me and my "baby" in the staging lanes at Quaker City Drag Strip.
That noticeable gap that we talked about on the passengers side door, may be nothing at all too.
I do know that Mustangs converts of that vintage did have a problem with "sagging" in the middle as time went on, and since they essentially used the Falcon chassis (which included the Comet) it may very well have had that chronic problem too. When they replaced the floor pans, it might have "stretched" or "re-arched" the chassis just ever so slightly, and it just showed up as a wider gap on that side of the car.
It's really to bad there isn't someone on the forum who lives near that dealer who could go over and check it out for ya.
I'd try to talk to them over the phone, and let em know your concerns. Sometimes you can get a better "feel" for their integrity talking one-on-one.
You can possibly negotiate a better price that way also.
If it turns out that they answer all your questions to your satisfaction, it's really not that expensive to devote a weekend, and fly down and check it out yourself. Then take a nice drive home on Rt.75. (I really do think the weather is going to break eventually).
If it's something that you think that you really want, then what better reason for a road trip?
I drove an old car back from Calif. once, and had quite an adventure. My best friend and I still talk about it 30 years later.