I started installing manual seatbelts that i got from pandagarage, and it came with interior trim. The trim is not a direct bolt on. You have to move some brackets to a spot lower (for the trim above the door where the passive track was) and even then you will see bare metal b/c the trim is not the same shape as the passive unit one. So you would need to get the 180sx roof cloth for it to look normal (or reuphoster it yourself somehow). Same goes for the piece between the two side windows.
So if you plan on doing this, find somewone that is just selling the seatbelt unit w/o the trim, you will save money this way.