Success! After much hammering, penetrating oil-ing, heating, and a bit of chilling, the dang thing finally came free, what a lovely little clunk sound. Wiggled the driveshaft around until I could get the drum loose and bolted it all back up, now just gotta go back and torque it properly to call it done.
I did not notice any significant signs of leaking from the transmission seal, which reinforces the fact I haven't had to top it up ever since changing it over to synthetic oil two years ago.
While it's fresh in my mind, here's a step-by-step for anyone else tackling this in the future.
1) Saturate the drum/flange connection with penetrating oil. Use an obscene amount. This can be done later but the sooner you start the better
2) Verify or make some kind of marking on the pumpkin end of the driveshaft to make sure the flange meets up to the exact same point. Mine already had a white line aligned with one of the bolt holes on both flanges
3) Remove fifteen bolts from the transmission end of the driveshaft and eight from the pumpkin end. You do not need to disassemble u-joints, but will need to get creative with extensions, u-joints, and wobble-sockets. A beefy impact wrench is your friend here, my Ryobi battery impact overheated itself but my Earthquake (HF) 3/4" breezed through it. Leave at least one bolt in the inner ring of the transmission end
4) Run a wire or rope through one of the empty bolt holes on the transmission end. You can't verify flange markings with the drum in the way, so this will ensure your transmission mating remains matched. Tie this off so it won't come undone, but leave enough slack for the driveshaft to move around a lot
5) Remove that last bolt on the transmission end
6) Knock the driveshaft free from the flanges and compress it if possible, then squirm under (blocking obviously) and wiggle the flange on the pumpkin end around until it's under the pumpkin. Doesn't hurt to tie it in place to keep it from sliding down to the ground
7)Apply a mixture of hammering, heat, cold, and penetrating oil to the brake drum until it comes loose. This may take a week of intermittent care
8 ) Wiggle the freed drum loose and down to get it out. Hopefully your bulkhead is already opened up to make it easier like mine was
9) You've probably got a rope or wire running through both flanges and the drum now, run a second one around the drum to make sure your mating remains matched, then remove the first so the drum can be thrown across the yard victoriously
10) Wiggle the driveshaft back into place, making sure the flanges meet the same boltholes they had originally.
11) Bolt it up using new hardware where available/applicable, fresh spring washers at a minimum. I'm using antiseize compound on mine.
12) (This part I have not done yet) Torque it down to spec, probably using a crowfoot wrench and applicable math as needed. The bolts are 7/16-20 Grade 8, torque to your preferred spec, I'm seeing 70ish for dry or 50ish for lubricated. Wire it up if you kept the castle nuts and are so inclined.
