The transmission oil level should be up the the level of the filler hole on the side of the case. The OD unit is not separately filled, there is no seal between the two and oil is shared through that cast iron collar connecting the two.

There isn't a relay on the OD unit itself, but there may well be on the wiring leading up to it. That changed over the years from the eraliest straight switches to later ones with relays that would reset every time, so you never got the 3rd-to OD shift you get when you leave the switch on. If you don't have 12 volts at the wire on the solenoid (key on, in 4th gear, OD switched on) then you'll need to trace that circuit backwards until you find where the 12 volts stop. And if there is a relay it could well be the culprit.
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I'm JohnMc, and I approved this message.