I have a Favor to ask, If you have measured the subframe assembly, and if you know the measurements, and if the rearend is centered in the subframe. Also if the axles are the same length, and if the lower control arms are the same length ? Kinda like I have illustrated in your pic, to me in the Pic it appears centered

Im contemplating, the use of the GTiR rearend and axles, but mounting them in a Miata Rear Subframe, because I have a B14 Chassis, and i think a double wish bone, or upper lower control arm setup will be easier to incorperate in my chassis.
The reason i dont wanta just use the Miata rearend is Its Ring&Pinion is 4.1 as to where the GTiR is 4.125, Theres 25 thousanths difference
And keep up the Good work, also will that drive line tuck up in the tunnel further or is that were it is to mount, just trying to figure where your gonna run your exhaust