The thread that Ken listed above shows lots of ways to fabricate an arm that puts the bellcrank in double shear. Whichever approach you take, I suggest replacing the top misalignment spacer with one that's welded to the arm. One less part to misplace and/or shift. My approach is shown below. Everything is made from 4130 and the 3D printed cap keeps dust and water from getting into the bearing.
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A few SL-Cs have had a weld on one of 2x2s crack after years of racing abuse. The ones that I'm aware of occurred in the same area - the upper bellcrank tube that connects to the front billet upright. To address any potential issue, I welded a gusset. Not necessary for a street car with some track use, but that's how I roll. A lot of heat is required to do this, so you need to strip the entire engine bay and fabricate a temporary cross brace between the billet uprights to keep things from warping.
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The best way to stiffen the rear tube chassis is to add the largest possible triangles to cross brace the corners. Both sets of billet uprights create a trapezoid (i.e., billet uprights, cross brace and billet piece under the transaxle) and the engine and transaxle make it difficult to cross brace the corners. The stock configuration has an indexable link that connects the bottom of the rear billet uprights to the rear suspension cross brace. However, the links are in generic location that provides room for pretty much any transaxle and exhaust. Not a bad generic approach, but it results in a shallow angle which isn't ideal. When running your exhaust, you should consider how you might move the tab on the cross brace to relocate the link and/or add additional links to maximize the triangulation of the corners.
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My transaxle has a billet bulkhead plate which allowed me to utilize it as a stressed member and cross brace the corners. Once the links are snugged up the rear becomes really solid. I took the same approach up front a tied the front billet uprights to the billet bellhousing.
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