I'd be interested in your solution. I had the same thought and put in s13 IRS. There are a miriad of issue you will need to overcome, so good luck!
Don't forget to post lots of pictures

Would be interested in pictures too.
You trying to fit it without cutting/modifying the original chasis rails?
Id be channelling the #### out of it if I went down that route. Chassis to chopped bumpstop clearance gets small fast when you bring them back down to earth.

Good luck, post lots of pictures plz

what about torsion bars? plenty of room for em

I think a good indendent rear end alternative after about 15 years of looking at rear ends (the car variety, much longer for the female ones) is to based it on a volvo 360glt which used to be leaf sprung and then when to indepedent.

If you have a look at these rare birds youll see what I mean for ease of design and the r200 or others could be used in this setup as long as the centre is secured on a custom chassis or cage.

MX5 is small enough to work surely. Apart from having a 4x100 studpatern which blows cocks, it'd seem like a pretty good option.

last time i looked mazda mx5 was 4 x 100. id go ae86 diff over mx5, but if you go s14, you could upgrade to r33 easily enough, 1600 guys do

Before Narrowing

After Narrowing

You can see the top control arm is touching the chassis rail, so i'm making an adjustable arm that will curve down and give the rail clearance. The pick up points will be the same. so it will not change the way that suspention is designed to work.

I'm picking up some coilovers tomorrow to see if they can be adapted to fit. I post some more pics.

You might already know this, but if you grab some 300zx turbo driveshafts you can machine them to fit. All other Nissan driveshafts (that I know of...but suspect GTR are the same) are going to be milled down at the point you will want to machine and too narrow as a result.
You chopped the irs a different way to me slightly and I built up the mounting point slightly from the chassis rail and added some support....but how low you go, wheels, suspension travel etc will dictate this of course.
Looking good so far!