Plans have changed!
After test fitting a Miata B6ZE today, and discovering a few things about it I didn't like, I've decided to do this right and keep the motor choice in the MB family.
While I won't be using an original 1.8 M136, I've instead decided on either a 2.3L M102 (there's a 5-speed manual on an M103 190E 2.6 that I know will fit the M102 at my local yard) or an M111.
There is far too little space for an M103 or any straight-6.
You can see that the Miata gearbox is rather LONG, and it sits back an ungodly amount:

MB boxes are shorter and I would likely have all the space I need with an MB I4. I think I'd be back in the same boat of not having enough space with the M103.
The other thing that makes an MB motor more appealing is that it dumps its exhaust on the right. The B6ZE dumps on the left, and even with a custom exhaust manifold may impinge upon the steering column. The intake on both the M102 and the M111 would sit higher and above the steering column, so no issues there. I'll be setting the fenders and panels in place tomorrow to get a better idea of how far I can move any motor further forward (have to consider radiator and fan spacing).
Anyway, some more photos from the test-fitting:


The front crossmember will need to be modified in some form, but I won't do that until I have the motor and gearbox I plan to use.
Progress is being made, slowly, but still being without an entire rear suspension has set me back.