Will our cage pass tech? (Thunderhill)

That looks pretty far back to me... But my opinion is worthless in this situation.

Email those pics to tech and ask them.

We read that guide before posting this, and it follows it from a technical standpoint, but visually it isn't super clear where they terminate.  We don't want to drive 600 miles to find out the cage doesn't pass tech because the backstays terminate at the rear edge of the rear wheels.

While not EXACTLY what they want to see, in my Lemons-experienced opinion, they will let you run with that.

IDEALLY they would want it forward of that by a foot or so. They MAY recommend for a second race to consider alternate ways of moving it forward, but Jeff and John will recognize that really is a sensible part of the car to affix it to since it is on the unibody rail and there's plenty of crumple/trunk area between it and the rear bumper. It all depends on the bigger picture ie does the rest of the cage look top-notch or is it lacking in some other area that more or less says "I know what their rules are but I know better than they know and I'm going to build it my way because my way is better and Lemons people dont' know their sh!t." For example, do you have a 1-piece diagonal and the harness bar connects to that, or do you have a 1-piece harness bar and the hoop diagonal is in 2 pieces? Are the nodes sensible at the roofline and the doorbars, or are they also an individual cage builder's interpretation of what Lemons tech SHOULD have said.

Stuff like that.

Because, let's be honest, in addition to lawyer speak and insurance regulations, there's a fair amount of d!ck-swinging that goes on in tech. As the car owner who is coming through tech, if you just put yours away and let the inspector say his or her piece and you respectfully listen and recognize their authority on all matters involving THEIR race, then you'll be all-good on backstays that look like that.

The perhaps unfortunate subtext is that first-time Lemons teams do often get a bit more scruitiny their first time through. And there may be someone in tech who has a bit of Mississippi Leg Hound in them. Heed this advice

If you added a bit more plate to what's there, maybe between the tube connection and the rear of the car, it would show that, at second thought, you acknowledged that what you have may not fit Lemons rules EXACTLY and that you made an earnest effort to get closer to their #sqin requirement and without the need to cut out and start over. 

Spreader plate extending rearward is fine. In the end, they want ample crush structure to absorb an impact before sending forces to the driver cocoon, if at all possible. They want to avoid the potential for scrambling an egg within it's shell, so to speak.