Your pictures I'm sure has made many of us very happy about the condition of our cars.

You might need to visit this site in the future:

https://www.millsupply.com/auto-body-rust-repair-panels/datsun/

12 hours ago, ArcticFoxCJ said:

I'll have this car so far apart there CAN'T be any more surprises...

When i saw your fender.. i knew thats trouble.. i hope the part thats UNDER the fender is not completly rotten away.. so show us the part under the welded fender..

Often that part has lots of rust as it is a water and mud catching part especially when you weld that fender shut...

So it was attached with pure bondo in big chunks.  No welding, thankfully.

What's underneath wasn't as bad as I feared, but it's still bad... especially on the passenger side.  

What I found truly bizarre is a bunch of crumpled mesh... like from a screen door... embedded in the bondo behind the passenger side fender.  I have no words.  

Both sides have been patched already. I would redo all of that. I suspect there is more concealed rust under those goober welded patches

Fortunately all of that metal is now available. Depending on how deep it goes.

Very common to see mesh or crumpled up newspaper behind repairs like this. The bondo needs something to help it fill the hole until it gets hard.

PO's

Got to love em...

2 hours ago, ArcticFoxCJ said:

I have no words. 

As long as you haven't run into thin heater duct sheet metal held on with wood screws you know your PO wasn't too bad.  It looks like you'll need both outer rockers at least, you may be able to save the fenders with repop fender patches, https://tabcoparts.com/16721.html

There are more expensive, more accurate patches but Tabco is not too bad.