Sounds like the classic driver error, put gear lever in neutral but forget to engage it. The flywheel is trying to drive the gearbox but brake is on, oil overheats, cooks seal and emits a fine white mist like smoke. The unwritten rule was, if it takes more than a pint to top up flywheel, pull the gearbox out. It is best to dismantle flywheel because I have seen them where the driven member has got so hot it has expanded and picked up on the periphery of it. Also the gasket may have suffered with the heat. If we were just doing the seal, it was not necessary to remove the engine, but if you want to go further that is your choice.
regards, Richard