For the differential/limited slip, I’m going with V8R’s Getrag Stage 2 kit which consists of everything to make a Getrag diff from a Cadillac CTS work in the Miata. That includes the diff mount, axles rated for 550 ft-lbs, full billet replacement hubs with ARP studs, and driveshaft. This is their same kit as designed for the V8 cars, but with driveshaft specific to the transmission. Opted for the aluminum driveshaft because race car.
Most of the swap parts are en-route right now, but the Getrag diff mount is already here. The two bare steel tabs get welded to the Miata rear subframe:
I’ve heard good things about the Getrag diffs. The V8R “spec” cars all run them in their L33 V8 swapped cars with good results according to Shandelle, and others have reported that they are an upgrade in behavior from the Torsen. Also good ground clearance (for flat bottom). If I turn out to not like it, I’ll explore other options.