Hello Roy,
Yes, you can rotate the gear as needed. For final installation, the most important parameter is that the rotor is pointing to the "slash mark" on the distributor rim, when the engine is ATDC #1, compression stroke. Be very careful to note that the slot on top of the timing gear is slightly off set to one side. The two cogs on the distributor bottom are also slightly off-set. Be sure to match them up correctly. They can be forced into place 180 degrees off if you are not being careful.
As far as correctly orientating the distributor, you can look at some original photos in the factory BBB to get the exact orientation of the distributor and its advance unit. The orientation is different with the early vacuum advance units and the later vacuum retard units. In every case the rotor should point to the slash mark on the rim at TDC #1 compression stroke. With vacuum cell orientation correct, you should have enough range to correctly adjust your ignition timing.