Narrowed Ford rear ends are the mostly widely available and would be inclined to go that way.

The Miata rear suspension is very good and all of the parts widely available, I wouldn't worry about the weight as it is the least of the issues. If the Miata rear sub frame will fit under the car, the next issue is going to,be how you resolve the bodywork and how are you going to widen out the front suspension to match. The track on a Miata is something like 6-7 inches wider than a 1200. The rear inner fender will need to be cut and rewelded to the bodywork to keep the structural integrity.

I went with an H190 on my car; the LSD unit itself was $1300, I picked up a 1200 project car with an H190 so that I would have all the little bits and pieces for,the conversion. I was able to sell of the left overs and cover most of the cost. I'd have to double check my numbers but I think all said and done it cost me $1500-1600.

You can probably find a complete Explorer rear end for $300-400 and then order a custom housing an axles. I'd still expect that to run you $1000-1500 depending on what deals you could find.

The H145 will survive a stock Mazda 1.8 is you are mindful of standing starts and how you shift. If you are going to Rallycross it and sort of wheel chatter will do in the H145 rather quickly.