Nice work Paigeo. The car is shaping up nicely.
Along the lines of Kiki's suggestion - since the bearings supporting the diff are closer to the bottom, that set of mounts will carry the greatest load (well actually the one closest to the sprocket will carry the greatest and the other one will probably be ~1/4 that just eyeballing your differential dimensions) so it would probably be easier to make those fixed and to make the upper ones adjustable. If you place the slotted mounts on the top, you can run a set of turnbuckles forward to the engine and they will always end up in compression (no need to worry about impact loads fatiguing the rod-ends and causing them to fail).
If you are not done yet, I would consider reinforcing those lower mounts to at least double shear 0.090" mild steel. Impact loads on solid mounted drive train parts are huge.
A picture is worth a thousand words - My Alma Mater did a nice job of eliminating some unneeded metal and simplifying the differential mounting on the 2011 car. Having a tube to support the upper mounts isn't really necessary of you direct the loads correctly.
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This is what you want to try avoid doing. The ends of the links are over contained so any flex places the links in bending. Plus the small (1/4" diameter I believe) unrated threaded rod is always in tension. Yea...it failed.
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I would also try to avoid making any of the differential mounts cantilevered off of the tube if possible. Keeping the loads being fed into the tube in tension/compression will probably keep your life the easiest.