Side cover leak

I have removed and replaced forks, with the trans in the tunnels of a 70 swinger and of a 68 Barracuda. So I know removing the cover can be done. As can just removing the bolts, suspending the cover, and putting some sealer in. But
What sealer sticks to gear oil?
My record for pulling that A833 is 17 minutes, which includes dropping the exhaust, and the Gear Vender, and the shifter, all by myself, allbeit on a 4-post lift, and with air tools.
So my question is this;
are you 100% sure that the leak is coming from the side-cover. This would be highly unusual.
The most likely leakers are;
1) the NSS switch which often leaks right where the connector pins exit the bakelite
2) the O-ring around the reverse pin where it comes thru the case.
3) the vent at the top of the tail, if someone forgot to install the baffle.

But yes, the cover gasket can leak if it was cut during installation by one of the shouldered bolts. ALL of the bolts as far as I know, go into blind holes. AFAIK only the bottom 3 shoulderless bolts get lockwashers.
Two of the shouldered bolts have longer shoulders than the rest, and they go in special locations, namely in the deep holes of the case which, IIRC are the center locations of the verical runs at the front and the rear. These two align the cover when they go in. If you install them elsewhere, two things will happen;
1) those two will bottom on the case before they tighten the cover, which, I suppose, would allow the gasket to leak in those locations, and
2) the cover is free to wander in any direction, by the difference between the shoulder diameters and the holes in the cover. This ain't much but, it usually allows the reverse internal interlock to bind on the cover and it makes getting into reverse a *****, if it goes in at all.
So, I mean, before you tackle the very tough job of trying to seal the cover with the trans in the tunnel, I would be absolutely sure that the cover is, in fact, leaking.
BTW
If your trans has a cast-iron case, that thing is a beast, IIRC 135 pounds, dry.
The Alloy cased version weighs less than my wife did on our Wedding Day, which IIRC was 105 pounds. What I'm saying is this, if you can do a push-up, dropping the alloy-cased trans is nothing. If you can do 5 push-ups, getting it back in, is only slightly more difficult.
But if you, like me, cannot slide your um "huge chest" under the car anymore and still get the alloy-cased trans onto your svelte belly, well; call over the neighbors kid who is always bragging how strong he is. That should shut him up.
If you take the trans down, here's a tip, find two bolts that will thread into the BH at the bottom two locations. Cyr the heads off leavin one about 3/4 inch long and the other can be a lil longer. Chamfer the cut lines and hacksaw-cut a screwdriver slot across the ends. make sure you can get a screwdriver into the slots. Before you take the trans down, install these pins into the BH on the bottom two corners. Install the shorter one on the passenger side. Then when you slide the trans out, it will come straight back, safely extracting the input shaft from the clutch, without bending the disc. Install it the same way except make sure the trans is in direct gear, and have something handy to turn the output shaft, which will rotate the input shaft, which will help the input gear into the disc. I use a spare yoke.
How-ever you do it;
Happy HotRodding;
this too shall pass.
One day you will have forgotten all the crap you hadda go thru to get a lil joy outta the old girl.