there are evap lines that run above the fuel tank I think. Given the codes and the rust in the rear, I'd be concerned about doing too much work to the tank and afterwards discovering it needs to be dropped and perhaps replaced. Of course, there's the chance dropping it now for inspection that everything up there looks good and the work was a waste of effort. I dunno what you can see under the access panels under the seat. maybe a 'smoke test' would be worth paying for?

Aside from the picutred rust, and as naive as it might sound, rust doesn't look too bad under the carriage or behind the panels.

 

I will try to post some more pictures. I really don't want to drop the tank.

Search 0420 spark plug anti-fouler "cheater". Installed as an extender for the downstream O2 sensor it fools the computer into seeing that the cat is working properly. Buy at Autozone in Help section. Drill out and install. Clear code and you should be good.

If you don't want to put the money into proper body work and paint, and aren't worried about the aesthetics of the quarters, there is a cheaper alternative that won't completely stop the rust but will majorly slow it down. You will need AUTOMOTIVE expanding foam (the home repair type draws moisture), 36grit sandpaper, "ONE STEP" rust converter (I don't trust the other brands), and asphault undercoat or a type of bedliner in a can. sand back past the blisters to expose all the accessible rust. knock off all the loose rust. Brush the one-step on all accessible rust and let dry. shoot just enough foam in the rust holes that is oozes back out. let the foam dry and trim it with a saw blade or box knife. spray the undercoat over all unpainted areas. DONE - ugly as sin two tone quarters that won't allow anymore moisture in.

PS. If you are any good with fiberglass, you can skip the undercoat, fiberglass patch the holes, and have a body shop paint the panel. I don't ever advise spray paint due to it's lacking a catalyst and not actually creating a proper moisture barrier.

Thanks for the advice. I got the CELs off months ago, and this car has been AWESOME! Why should I be suprised? It's a subie  So now it's time for phase 2----(What Fubaru said)

 

I'm in AZ. so, not too worried about rust spreading, and I've been told the car is safe. Going to fiberglass the holes, and paint over the car. Hope to add before and after pics. 

Oh, and in case anyone has similar problems, here is the much delayed update:

 

Well, figured I should update, even if it's months late.

Got all CELs off her for under a $50 bucks! BWAHAHHAHHA! Got a lot of help from you guys, and a little luck.

1-
Fixed code p0325 by replacing the knock sensor for 15 bucks on ebay. 

2-
Fixed code p1400 with this http://forums.nasioc.com/forums/show....php?t=2310341 ..I ended up running a new wire from the computer under passenger carpet to behind the charcoal box in the rear. 

3- Fixed p420 (temporarily). The catalytic converter is bad, but I can still get it to go for 100 miles or so before it turns on again, and got it past emissions. 

The 2 others were associated/repeated codes. 

With the brakes done, it has been a super reliable ride for the old lady, and I think I'm going to tackle the rust and try to plasti dip it! Will update it.