Interesting to hear you have an alloy centre from a 310. You have a 1200 sedan right? This would mean it's a H145 going by it's torque tolerance.

The diff centre will suffice provided you don't hammer it too much. I had a heavily worked A12 that butchered the diff centre once I resorted to some 13 x7 watanabe's- this was because the larger traction surface area of the watanabe wheels transferred most of the shock back through drive line. So, basically if you stick to narrow profile wheels the only thing that will break is traction ( = wheel spin).

The gearbox will be fine, it will only fail after repeated abuse (ABUSE) and I would bet that the diff centre would fail before the gearbox did.