So this ended up being super fun and I really don't know how many more times I need to learn the lesson of doing things right the first time lol
(hopefully none)
How to fix your fuel leak!
1. Notice that your car is dripping a really frightening amount of fuel from around the DS rear tire
2. Become horrified when the true scale and source of the leak is discovered (lots of fuel weeping out of the flange where the filler neck seals to the tank)
3. Attempt futilely to use an entire tube of Seal-All to stop the leak without draining the tank
3. Drive to the closest auto parts store and buy as many holding containers as you can afford
4. Drain all the fuel out of the tank (that you just filled hours before noticing the leak, of course)
6. Cut out redundant HICAS hardlines so you have even half a chance of fitting your hand in there
5. Attempt futilely to use an entire new tube of Seal-All to stop the leak (with the tank drained)
6. Refill and notice immediately that Seal-All did nothing
7. Drain tank again
7A. Order $20 o-ring (+$8 shipping)
8. Patiently wait for o-ring to arrive (and spend a couple hours peeling remnants of dissolving Seal-All off the flange)
9. Remove the flange and realize that 3 out of 6 bolts have the threads ripped out -- hmm yeah that could do it.
10. Install new o-ring and replace bolts -- be really careful! These bolt holes are capped at the end (forgot the technical term) and if you puncture them you need a new gas tank. I didn't hear about this until after I'd already reinstalled the flange but I followed the FSM torque specs of 3.8 ft/lbs so I should be in the clear.
11. Put gas back in the car and cross your fingers! (I have 12 gallons in the car right now with no apparent leakage, and another 6 gallons in a container in the garage that needs to go back in too)
I've heard conflicting info on this so I'll give it a brief note here -- some sources say that the o-ring is actually 1993+ only, and my 1990 did not have an o-ring installed upon disassembly. I had one guy tell me his 1989 JDM Z32 did in fact have an o-ring and a groove for it on the flange. I did not have a groove but with some vaseline and a bit of care I was able to get the o-ring pretty well centered regardless. I will update if this repair ever gives me problems again but I'm hopeful it's all buttoned up