Dave Bivens Mirage Build

That panel across the underside of your chassis will improve its torsional stiffness, provided that it is securely riveted or welded in place.

I agree. I added a .125" aluminum sheet under everything forward of the engine. I have another piece for under the engine, but with all the heat in the back, and wanting to gain full visuals of the mechanicals back there in time of need, I chose to leave it off. Yeah, it was about 45 lbs, which accounts for one of many little things that have made the car stretch well beyond 2100lbs. What helps offset that angst, is added peace of mind in protecting the underside of my britches, a smooth, raked underbelly for some downforce, added chassis rigidity, and a barrier to water (not that I drive around in any adverse weather).

I'm sure you've thought of this, but will say it anyway. Don't forget to drill and occasional small drain hole for any fluid that may find its way onto this panel (specifically coolant). I drilled mine immediately next to the framing corners so that normal forces of driving push the fluid toward the holes, and makes it detectable if it's a slow leak.

Drain Holes, very good idea! thanks

Adding more build details, wasn't happy with how the front frame that holds the radiator and from clip attaches to the main frame, so I added a bit of strength, triangulated the mount, the way the radiator is mounted is also visible, ignore that pipe coming out of the radiator, that will be changed of course.
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Here's how the front body section gets secured to the front frame, I added a fibreglass framing piece, glued and glassed to the inner section,
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and here the bolt thru the frame is visible, ignore the tape measure,
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ok, it's freezing outside so time to add a few more pics on progress, more on the front body section mounting, here's what the frame section looks like, I added a piece of tubing to receive the bolt threaded through the lower front section of the body (pics above) and also boxed it in,

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On to the rear hatch, this made me nervous, when it's open it really bounces on the lower attaching point and where the metal mount is glassed to the rear hatch just didn't seem strong enough so I added more glass, used a double biaxial weave with mat on one side, works well for complex shapes.
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I also added reinforcement framing going from the pivot point to the top section. I added glass to the top section before adding the square tubing. Used construction adhesive to affix the tubing to the underside of the rear hatch and also the adhesive provided a good transition from the metal to the hatch for the fiberglass I added over the tubing, pics can explain better than typing, when the hatch is closed the added support tubing lines up pretty close with the frame members going down to the pivot point.
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Also, for better body alignment, I had to move the rear hatch pivot point higher, can't even see the old hole, welded in nicely, (sorry the picsare rotated, not sure how to correct that, they correct orientation in the file folder)

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