First let me say that it is difficult to measure parts, in the shop, with great accuracy when the features are offset to different planes, as most of the Miata rear suspension parts are. Nobody should expect perfection from those who went to the trouble of attempting it, and providing this valuable STARTING POINT information. There are bound to be some errors, and you should allow yourself some correction method or adjustment point when constructing. Know that if you finish weld your entire frame and all the suspension brackets before you have donor parts of your own to mock up... you may have a problem. The exception is if you duplicate someone else's design exactly, and they assure you it was within adjustment range to get the camber/caster/toe right.
I laid out Chet's subframe, LCA, and upright dimensions at zero camber as shown below, and ended up needing an UCA of 8.875" length rather than the 8.375" he measured. That's not to say his UCA dimension is wrong; it could be the 5.875" or 4.5" on the upright, or the camber adjustment may have been set to one extreme, or a combination of a few small errors that combine to make up the mystery half inch. If I had to build a frame just on the available info here on this post, I'd probably leave off the UCA mounts, and allow enough space to move them inboard/outboard a bit. UCA brackets are lightly loaded, so don't have to be right at a tube/tube junction.
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