Stall speed is the rpm at which the maximum torque multiplication happens. It has to do with the internal design of the fins and the angle at which they are at. It should be matched up with the torque peak of the engine. That being said, it varies a lot with the input torque, as BudW pointed out. It is one of the main specifications. The higher the stall speed, generally, the smaller the diameter of the torque converter.
Lockup torque converters are more efficient than non-lockup converters when they are locked up. All non-lockup converters have some losses because they are relying on the fluid alone to move the car. A lockup converter uses a mechanical device (sprag clutch is the name, I think) to mechanically join the input and output shafts, eliminating the losses of the non-lockup converter. This is only during steady state highway cruising, and has the added benefit of slightly lowering the transmission fluid temperature, and engine rpm as well. The efficiency increase is not huge, but it is noticeable. It would be anywhere in the 5-15% range in terms of gas mileage. A lot of peformance oriented people don't like the lockup converters, as they believe that the lockup mechanism is prone to failure. For street cars, it is a benefit in many ways.
The torque converter is not unbalanced, the 360 engine sort of is. The 360 is designed to be in balance with the torque converter and flex plate/flywheel attached (i.e. externally balanced). Weights are added to the flexplate or the converter to bring the entire assembly into balance. The 318 is internally balanced, which means without the flex plate/flywheel or the converter attached, the engine is in balance. So all 360 torque converters/flexplates will have extra weights added to bring the entire assembly into balance. The 318 converters may have weights, but only the balance the converter, not the entire engine assembly as well. The 318 converter will have fewer and smaller weights. The weights to balance the 360 can also be added to the flywheel alone, permitting the use of a 318 converter. B&M for one makes a flexplate that balances a 360 properly. I believe that the factory used weights on the converter for 360s.
Torque converters, even if they are round, can have imbalances, just like wheels can. Wheels are round, yet they need to be balanced properly. Torque converters are the same, except that the standard 360 torque converter also has weights added to bring the engine into balance as well. You can think of the 318 converter as being balanced on a balancing machine. The 360 converter is like a wheel being balanced on the car, where it balances not only the wheel, but the disk brake rotor as well; in the 360 case it balances out the engine as well.
All that being said, you can, with the proper machine work, internally balance a 360. It involves adding denser weights onto the crankshaft, and is usually done during an engine teardown/rebuild.
If you use a standard 360 torque converter/flexplate on a 318, you will get severe vibration, as the added weights on the 360 torque converter/flexplate will throw the 318's balance off. If you use a 318 torque converter on an externally balanced 360, you will also have severe vibration, as the added weights to balance the engine are missing.
The drain plug on the converter is a convenience when changing transmission fluid. The converter is full of transmission fluid, and when changing transmission fluid, to get the fluid out of the converter takes a flushing operation without a drain plug on the converter. With the drain plug on the converter, you just open up the drain plug and it runs out.
