I welded the frame rails to the rear bulkhead, adding some diagonal TLAR* tubing. I'm thinking about not adding all the rear body work...leaving the arse end wide open, something like an At-om.
I'm also thinking of putting an honest-to-goodness rear bumper on the end of the Miata frame rails, just like on a Miata. Not a Miata bumper, but a little tiny one like a Geo Metro... We'll see.
The subframe remains a bolt-on assembly. If I had a clearer vision and more confidence, I would likely weld it in place...there's no reason for it to ever come out, I believe. As it is, it's proved convenient to remove it when flipping the frame and whatnot.
I'm also considering not doing the underbracing that other folks have added under the lower suspension pickups. The Miata has it all hanging from above, after all. Then again, it has some pretty beefy frame rails that tie into the whole body, so I probably will add lower supports. There's little reason *not* to, I suppose. Again, we'll see.
There will be more bracing added later on, not shown here. Most notably, I will have the rear rollbar supports land on the frame rails. This will provide huge support for the frame rails, and also give those rollbar tubes a much better (flatter) angle than most of the Locosts I've seen, where they tie into the bottom of the rear bodywork support.
On my next car (next car?), I think I'll extend the rollbar hoop all the way to the floorboard, replacing the vertical 1" square tubes at either end of the rear bulkhead. Heck, I might cut up what I've got now and do that...but probably not. I hate backtracking!
* TLAR - That Looks About Right