The collapseable column came mid-year in '67, so if you get a late production car (or one with the tilt-away) it should be collapseable. Failing that, give Flaming River a call.
I don't have the actual measurements, but the '67-'68 has about a 2" wider track, which would be a benefit to handling, but in purely stock form there probably isn't much difference. You should also be able to stuff larger wheels and tires under them. So I'd say they have more potential, but Shelby had them all handling pretty well so I'd go with what appeals to you.
Oh, and like a previous poster said, the 69-70 is a little bigger (and raced in the Trans Am series), so the same thing applies to them vs. the 67-68.
Personally I don't care for the 71-73, I'm sure they can be made to be very quick and handle well (e.g. Boss 351) but they just look bloated and mid-70's Camaro-ish to me. Just my personal taste.