Mike B - If you overfill it, won't there possibly be leakage, at least from the input shaft end? Not sure what you mean by "as far as you can"??? Many/most of the SM420s didn't have full seals on the input shaft, just the "slinger" set up which is going to at least allow seepage until the oil level falls below the bottom of the input shaft.
Another suggestion for long term storage is to seal the shift tower. I've actually packed the shifter ball top and tower opening with grease and then covered with plastic wrap and rubber bands. Leaving this open to the air, even in an enclosed storage, still allows for moist air to get inside the top of the transmission and eventual condensation and rust forming. It can significantly affect the shift tower components over time. This is for really long term storage or if there's potential for leaks in your storage space from rain and what not.
