Help with rear differential size

[LAC99]

Hi all. I´m working on a 84 levin that originaly had a 3.90 rear differential with no LSD and drum brakes, so as you can imagine I need to change my entire rear axle. I have two options right now but I have some doubts that I hope you can help me with.

7.5 inch rear diff with LSD and 4.37 final ratio with disc brakes (i think is from a nissan pickup, already modified to fit in the ae86).

6.7 inch rear diff with LSD and 4.11 final ratio with drum discs (from a 82 or so corolla). Of course I would have to fit in disc brakes later if I choose this one

My biggest concern is about the size of the differential, I don´t want to lose power if I choose the 7.5 inch diff. Have any of you adapted a big differential like this one? did you lose any power?

by the way, my engine is a 20 valve 4ag (silver top)

thanks!