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If you have a spare clamp you can put that on your second distributor. Eyeball it so it's physically in the same position as the one you have on the engine. The notch in the rim should match the position of the one on the car and that will position it within a degree or so of that one. When you pull the original one, doesn't matter how the rotor is positioned you can just insert it so the offset drive drops in to the same position. With the clamp adjusted and tightened the single attachment bolt will position it correctly. Adjust clamp if needed. Run engine with the second one fitted and check timing. Should be close but you may need to slacken off the clamp for a minor adjustment. Maybe run that for a while to prove it. Keep it in or swap back to the previous one. That takes longer to write than to do the swap.