To clarify, back in 1983 when the T5 was introduced into the Mustang lineup, Borg-Warner did not call the T5 transmission non world class. Rather, the term "Non World Class" came about only when in 1985, Borg-Warner released the T5 globally, under the name World Class. From that point on, there needed to be a way to distinguish between the early model and late model (World Class) T5 transmissions. Therfore, the early models began being referred to as the Non World Class transmission. (Just like there is a HO 5.0L V8, and a non-HO 5.0L V8)
Internally, there are some differences. First off, as mentioned above in the breakdown sections, NWC T5's used a 2.95:1 first gear and a 0.63 ratio overdrive, whereas World Class versions used a 3.35 first gear and a higher 0.68 ratio overdrive. Synchros changed from solid bronze to fiber-lined steel, needle bearings were installed on the mainshaft cluster as opposed to the solid shaft design of the Non World Class T5, and finally, in 1990, nickel content was increased making the gear set in the 1990-1993 Mustangs the strongest ever. In a nutshell, T5's produced from 1983-1989 were rated at 265 ft-lbs of torque, and 300 ft-lbs from 1990-1993.
Are the NWC and WC Transmission Interchangeable? Yes, both transmissions are interchangeable. There are no mounting differences or changes in the case that would prevent one being swapped for the other.
Which is better, the NWC or WC T5? Purely from a power aspect, the World Class transmission from 1990-1993 is the best, as it is rated at a capacity of 300 ft-lbs. However, some owners who have used both versions of the transmission reported the NWC version as shifting smoother in stock form.