But when researching the full level on a 4L80E, they all say 1/4" above the mating surface of the case?
"They" are missing an important detail in there - it's even with the pan lip, or UP TO 1/4" higher. The _correct_ procedure is to check with warm fluid, with a full level even with the pan lip. But if checked while hot, or if your driveway/road/whatever isn't perfectly level, it can be up to that 1/4" higher mark. Of course, everyone's idea of "warm" vs "hot" is going to differ too, lol, and I don't recall off the top of my head what GM's spec is now on temp. I've just always done 10-15 minutes of drive time around the block, and called it good.
3/4" down seems like you'd be running a lot closer of being too low...there's only about an inch between the low and full marks on these sticks, which equates to about a quart, and I know from experience my trans will start slipping pretty bad at 2 quarts low. I'm _guessing_, perhaps, that his recommendation was based on it being a 4x4 trans, and is trying to avoid too much fluid from getting up into the gear train at extreme angles, and causing oil aeration issues?
The other side is that with a deep pan, along with a filter sitting deeper, you can get lower levels in the pan without running into the risk of uncovering the filter. Not exactly the kind of situation that would give me the warm and fuzzy feelings, and not typically the reason for running a deep pan, but it _could_ be run like that.
I build all my own transmissions, but I'm not trying to pass myself off as any kind of pro trans guy. Everything I've ever seen/heard on these GM transmissions has been the same - full fluid level is even with the pan lip.