The Mustang you're working on sounds like a lot of fun.
I like 4.11's and blowing off the tires well into 2nd gear, up to 60 or 70 mph. I know that's not how you go fast, but shoooo-ey I like it anyway.
BUT, downshifting into 2nd in the sweet part of the powerband, at 75 mph and shifting into 3rd at 115 mph sounds like fun, too. It's nice to always have more speed than you need, as opposed to thinking "shit, I'm about to run out rpm" in top gear. I can only do about 135 mph in my Cougar and that's over 7000 rpm. It is fun, but you're there and you're done. Once your done, all you can do is let off the gas.
So, I am thinking about something different. A Gear Vendors overdrive maybe. Or just high gears in the back with an appropriately cammed engine?
I think 3.25's are about as low (high numerically) as I would go for a true highway car. I'm not sure if I want to go too much over 150 mph in an un-aerodynamic old car, anyway. It's just too creepy. I can handle cruising at 3000 rpm all day, but 3500+ makes me feel like I'm beating the engine to death. It just takes the fun out of it. So the 3.00 to 3.25 range seems like the place to be.
I have a few Ford 9 inch gear sets in this range so if you want one in that range let me know. I'll send it to you.