To minimize the driveshaft angle, assuming that it turns the correct direction as-is, after narrowing the miata subframe as necessary, offset the diff housing to use the stock axle on the left side and make a new shorter axle for the right side. You may need to limit suspension travel to keep the tripod in the cup on the right side. Roll a 6' x 12" x .050" sheet to a 3" radius, then trim the floor as necessary to fit the new tunnel.
Thanks for the input. Yes, the motor does turn the correct direction. As to the driveshaft angle, it's not only offset, it looks like it's going to be too high - I have the "double whammy compound angle". In talking with Jesse at High Angle Driveline, he says that there's a high probability that unless I get one plane lined up, I'm in for vibrations at highway speeds.
And in keeping with what is likely the going to be the most repeated phrase during this project - "Nothing is ever easy" - the ST has a 2.833 final drive so if I go with an offset diff, I need something with an appropriate ratio. Currently leaning toward a 3 series BMW that has a 2.93 - see pic.
And here's my first question: If I have different axle lengths, is there really nothing to worry about beyond binding points of the related suspension and axle pivots? No other negative impacts to consider?
If I opt to go non offset diff, the other option is to develop a gear/chain offset unit. This would open the door to 'correcting' the primary gearing of the engine and I could spin the output of the offset unit up to whatever I wanted. Now I can run whatever rear end I want. (I like the Subaru rear ends as they are quite narrow which is key for this build.) Now, I have yet to research what this approach does in terms of impacts to torque numbers and all that, but I'm kinda leaning this way.
All comments and help welcome!