1984 12a differential options?
 

Hello everyone!
I am restoring a 1984 12a RX-7 on a budget, and while surfing on this forum, I've come across some threads regarding changing the inner parts (not sure) of the rear differential to achieve shorter gears (I'd like something around 4.2-4.8).
This got me wondering, does this apply to the GSL-SE only, or can I do this on my open-differential'd GS?
I'm particularly interested in information about Kia Sportages and Miata's, as there are plenty around - what year and what engines should I be looking for?

Thank you for the help!!