[Nuttyman54]

5803 is running 2 GEM gearboxes with MiniCIMs in a 200:1 overall reduction for a similar weight and length arm and they've been fine for us so far. The GEM should be capable of handling the load as long as you keep the sprocket close to the base of the shaft (which it looks like you are).

I would be concerned about the amount of torsion going through the hex shaft though. It might be OK, but I would recommend trying steel hex there if that's not already the plan. Aluminum is likely to start yielding there, if not from the dead weight than from the shock loading of the arm bouncing while you drive around. If there's any way to flip it around so the sprocket is bolted to the arm structure and the axle is a dead axle and free-spinning, that would alleviate the concern too.

I can't tell from the picture, are you using #25 or #35 chain there? I would highly suggest #35 for this application, the loads are high enough (especially if the MiniCIMS end up stalled driving the arm into something) that you will probably break #25 chain.

Best of luck! Arm joints can be tricky to do right, especially with a heavy load on the end.