Torsional frame flex reduction

[exwestracer]

This one?
You could get by with a lot less triangulation. That center section is MUCH more rigid than the attachment points... But, yeah, it's the right idea. Personally, I like the "semi-monocoque" approach Lotus used with less tubes and the sheet metal structure running full length. That also gives you the option of just painting it and making that the interior tunnel surface. When you tie back into the frame, try to get as close to the front and rear suspension as possible (at least get ahead of where the frame widens out). Try to get as much beam height ( 5" rect tubing stood upright?) as possible with the pieces that tie back to the frame. You can tie the rect tubing to an upright piece of round or square at the ends of the center "spine".