Bobber,
I'll just reply here and keep it short, only because I'm going to be adding pictures and references with my measurements, and I think they add to the substance of the build log.
To interface with the subframes I built 8 'spools'. Each is made of 1.75" round tubing with .134 wall thickness (heavy stuff), plated with 1/8" on each end, and with a 1" (.083) compression tube welded to the plate on each end. The 1/8" plate was highly beveled before welding, and then both the compression tube and weld bead were ground flat. Here's a picture of a trial spool I made to see if I liked the idea:
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The four spools that interface with the rear subframe are 4.25" tall. Four inches of tube with an eighth of an inch plate on each end. You can see the basic setup here:
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As you can see, the spools are connected with 1.75" tubing (.095 wall, in this case, as I had some leftover still). The tubes on the sides are pushed up as high as I could get them on the spools, so that they give adequate clearance for the half-shafts coming out of the differential. This clearance is the reason the spools in the rear are a full inch taller than the ones in front. The tube in the rear connects the rear spools and is pushed up as high as possible with deference to the side tubes. There is no tube connecting the front spools on the rear subframe, since the transmission is in the way.
The four spools that interface with the front subframe are 3.25" tall, otherwise built exactly the same way as the rear spools. The tubing connecting the spools is also 1.75" with .095 wall, and similarly has the side tubes pushed up as high as possible and the front tube bumped up to the limits imposed by the side tubes. Of course, there is no rear connection, as the engine is in the way.
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The stiffness of the subframes and the nature of how I interface with them is, I think a critical aspect of what will be the functional stiffness of this chassis. After studying the subframes and how they are built (rather thick cast aluminum, by the way), I think they will be as rigid as anything I can build to mount to them, as long as each point of interface is not allowed to move in relation to the others on the same subframe. In other words, I think the subframes are extremely stiff when bolted to a stiff structure, but they were engineered assuming as much. GM had them bolted directly to stiffened points on the very-large, hydroformed, frame rails. There were no bushings, and the bolts went through the frame rails (and 1" compression tubes, which is where I got the idea), but the steel plate GM used to contact the subframe mounting points is not as thick as what I'm using, or as directly tied in to the next structural level. The spools I've built contact the entire interfacing surface, so there would be nothing to be gained by making the bases of them larger.
I believe that the weakest point of the way my tube structure interfaces with the subframes is the lack of bracing between the front two spools on the rear, and the rear pair in the front. To be kept from bending inward, toward one another, the pairs of spools will need to be triangulated to the nearest hard points; hard points being my name for nodes already fixed in three dimensional space. The hardness of the point is a function of its fixity.
In the rear, the hard point will be the middle of the harness bar; this will be a very hard point. In the front, I think the hard point will be the bottom of the firewall triangulation; this is less hard than the rear, and I may look to either make the point harder somehow or double up by bracing to a second hard point.
I don't expect anyone to actually read most of the above, by the way. I'm mostly putting it down for my own build record.
-Graveyard