Rather typical of doing essentially a one-off, there is a constant need to evaluate and plan for the effects of various decisions on other things down the line. I've always planned to brace between the lower control arm tubes near the bushings. Some time back I came across an analysis of the effects of doing so, and it was substantially beneficial both in terms of rigidity of the lower control arm plus less twist of the control arm tubes under braking. That is more braking force actually absorbed in the chassis and suspension rather than the tubes themselves.
However, where you place the cross brace will have a direct impact on what your side body panels must look like. The Haynes Roadster book specifies an inward bend of an approximately triangular area in the side panels. That makes it easier to place the cross brace closer to the 2 lower control arm bushings. But most people don't do their bodywork that way. They do it like Champion did on page 105 of "The Book." I actually like that approach better myself.
Dropping a plumb bob down from the upper chassis rail to the build table, simulating a Champion-like side panel, that puts the cross brace out about 3-3/4" inches from the outer surface of the rear, lower bushing. Too far, I think, at least it will lessen its effectiveness versus being closer to the bushings.
Here is a finished car off the Internet that uses the Haynes Roadster bend, and also a cross brace. You can see it puts the cross brace much closer to the bushings, which I think is preferable.
Attachment:
Haynes front bend-in on side panel 1.jpg
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Haynes front bend-in on side panel 2.jpg
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Haynes front bend-in on side panel 3.jpg
For me, the benefits of a better lower control arm outweigh the aesthetics in this particular situation since the wheel and tire do cover the open area pretty well. I think I'm going to do as this builder did. However, I had to reason this all out and decide before I finished the LCA design.
Cheers,