Here is my opinion - you will be hard pressed to find a more decent human being, and someone more knowledgeable about Toyota products than Evan - @fj55-100 . Anyone who can graft a 2UZ and associated 100 series drivetrain into a Pig body has some serious skills. And he is as nice as the day is...
It sounds like you are on the case.
I hope it turns out to be what you suspect.
I have had 2 tanks start leaking on me with the vent lines fully hooked up, so I hope you don't have my luck!
Dropping those tanks and stabbing a repaired unit in does not rank high on my list of things I want to do...
QTY 6?
Two spares?
I am sure you have a plan.
My guess is those Toyos will age out before you wear them out.
You can probably manage w/ a single spare.
You need to make sure you can get a pinion w/ coarse spline if you want to use your old flange.
The change from coarse to fine spline happened somewhere in '78 (after my 09/77 40 was made).
Otherwise, if all Yukon sells is fine spline pinion, check w/ Kurt @cruiseroutfit , they have all manner...