1. 1.8 open converted to RX7 LSD:

Parts:
94-05 1.8 Miata open assembly
84-91 NON-TURBO Rx7 clutch-type lsd (89-91 is a viscous, don't bother)
94-95 Miata stub shafts or 86-88 RX7 stub shafts and 94-95 Miata half-shafts
or 96-05 Miata axles (no stubs needed)
RX7 Circlips (fit onto the end of the stubs/axles, miata clips will not fit)
94-05 Miata driveshaft
Miata Ring and Pinion

Proceedure:
Open the 1.8 miata & pull the open diff. Decide what ring and pinon to use (easier to use the miata gearing, costs more $$$ when having a shop set both the pinon depth and backlash). Bolt the correct ring gear to the RX7 LSD. Drop it in, set the backlash (and pinon depth if swapping pinon gear), and reassemble the housing. Swap/install as normal.